Chapter 4 The Science of Driving
Suppose you would like to buy a car for daily use. To decide which car to buy, you read many leaflets about different brands of car, like the one shown on the right. However, you probably get frustrated with those keywords like horsepower, displacement, V-6 engine, ABS system, ... etc. You don't know which car is the right choice for yourself!
In fact, car and driving has a close relationship with science especially physics. For example, when people talk about time of "zero to sixty" of a car, they are actually considering the time for the car to accelerate from rest up to the speed of 60 miles per hour. In this chapter, let us study about car and driving by scientific analysis.
Mechanics, the branch of physics investigating motion, plays an important role in understanding about the motion of car. Thus we can use the concepts and tools developed in mechanics to investigate the motion of car. In fact, physics can tell us how cars react with different types of forces and thus we can make predictions about the motion of a car. For example, we can study whether a car would turn over if it moves on a circular track with certain radius. Physics is also useful for the investigation of the cause of many car accidents. Therefore, physics is indispensable in the study of motion of cars.
| Preliminary knowledge |
- Keywords describing the motion of a car
Typical Values of Acceleration [i] | Objects/Events | Acceleration | | Elevator (fast service) | 2.9 m/s2 | | Space Shuttle (take off) | 29 m/s2 | | Football tackle | 390 m/s2 | | Jumping flea | 1960 m/s2 | | High speed car crash | 6860 m/s2 | In the terminology of mechanics, a car moves with certain velocity when it is moving on the road. You may think that velocity simply means the speed of the car. Indeed, velocity does not only tell us the speed of the car. By definition, velocity refers to how fast a car moves towards a particular direction. So, if two cars move with the same speed towards different direction, they are moving with different velocities. To change the speed of a car, you push the accelerator of the car. In other words, the car accelerates if you push the accelerator. In fact, a car accelerates whenever its velocity is changed by either changing the speed or the direction of the car's motion. So, even if you are driving your car at a constant speed while turning around a corner, you are still accelerating. The acceleration of a car is a measure of how fast the velocity of the car changes in a given time interval. When we speak of the velocity and acceleration, we usually refer to the instantaneous velocity and instantaneous acceleration. For example, the speedometer of a car tells us the instantaneous velocity of the car; that is, it shows the velocity at that instant of time. If you inspect the readings of the speedometer at regular time intervals, you probably notice that it changes often.
The instantaneous velocity of a car often changes! [ii] By definition, velocity refers to the distance traveled in a certain time period while acceleration is the change in velocity in a certain time period. If we calculate the velocity and acceleration over a very short period of time, we will obtain the instantaneous velocity and instantaneous acceleration. However, if we calculate these quantities over a very long time interval, then we will find the average velocity and average acceleration. | (Courtesy: PhysicsNet) |  |  | - Accelerations
The units of velocity and acceleration: Velocity is usually given in terms of kilometers per hour (km/hr), miles per hour (mph) or meters per second (m/s). Since acceleration is equal to the change in velocity per unit time, acceleration is usually given in units of km/hr2 or m/s2. Another unit of acceleration that is commonly used is "gee's" or simply "g's". You probably have heard about this unit in the discussion about space travel, combat aircraft or racing car. For example, the maximum acceleration of a combat aircraft is typically around 9.5 gee's. One "g" is 9.8 m/s2 and it's the maximum acceleration that your tires can withstand. The ability to accelerate, especially the ability to accelerate from 0 to 60 mph, is one of the typical measure of the power of a car. The table below shows the times taken for several 2002 cars to accelerate from 0 to 60 mph. Time from 0 to 60 in seconds for several models of 2002 cars [iii] | Types of Vehicles | Model | 0 to 60 (sec) | | Family sedans | Ford Focus ZTS | 9.6 | | Hyundai XG300 | 8.9 | | Honda Accord EX V-6 | 7.6 | | Sport sedans | Ford Thunderbird | 7.0 | | Porsche 911 GT2 | 4.1 | | Mercedes-Benz E430 | 6.3 | | SUVs | Ford Explorer | 8.0 | | Jeep Liberty | 10.0 | | Toyota Highlander | 8.3 | |
You have to push or pull a car in order to accelerate it. Scientists call this pull or push to be a force and the force is supplied by the car's engine. The term "force" was defined by the genius physicists Isaac Newton more than three hundred years ago. He formulated three laws of motion based on his findings about the properties of force. Newton's laws of motion is able to explain and predict how a car moves in different circumstances. Let us review the Newton's laws of motion one by one. There are three laws of motion due to Sir Isaac Newton.
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|  | | Sir Isaac Newton [iv] | - Newton's first law of motion
Newton's first law of motion states as follows. An object continues in a state of rest or uniform motion in a straight line, unless a net force acts upon it. In other words, all objects tend to "keep on doing what they're doing" unless acted upon by a net force. What does the term net force (also called unbalanced force) really mean? Let us consider the following example:
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| (Courtesy: The Physics Classroom ) | Since the two forces acting on the book are of equal magnitude and in opposite directions, they balance with each other. The book is said to be at equilibrium. No net force is applied on the book and thus the book maintains its state of motion. When all the forces acting upon an object balance each other (i.e. no net force), the object will not accelerate. If an object is at rest, they will continue in the motionless state. If it moves with velocity of 10 m/s towards South with no net force acted on it, it will continue in this state of motion (10 m/s, South). If it moves with velocity of 20 m/s towards right with no net force acted on it, it will continue in this state of motion (20 m/s, right). The state of motion of an object will not be changed as long as no net force is applied on the object. All objects resist changes in their state of motion --- they tend to "keep on doing what they're doing". Such resistance to change in state of motion is called inertia.  | |  | | (Courtesy: Mechanics - Team 28388) | | (Courtesy: The Physics Classroom) | At the first sight, it might seem that the first law of motion is contradictory to our common sense. According to the first law of motion, a force is not needed to keep an object in motion. However, we may observe that a moving object will eventually come to a stop even there is "no force" (really?) acting on it. In fact, the object is slowed down by a net force named "friction". In the absence of frictional force, the object would continue to move with the same velocity on and on! That is to say, force is not required to keep a moving object in motion; indeed, a force is needed to bring that object to rest.  |
| (Courtesy: The Physics Classroom) | Newton's first law of motion tells us that all objects resist changes in their state of motion. All objects have this tendency --- they have inertia. Is there any object having more inertia than others? Yes, of course! Mass is a measure of the inertia of a body. Roughly speaking, mass is the amount of material in a body. A body with larger mass has more inertia and thus it has larger tendency to resist changes in its state of motion. The mass of an object is usually given in unit of g (gram) or kg (kilogram). - Daily examples of Newton's first law of motion
- You probably have experienced inertia in an automobile when it is braking to stop. Although a net force is exerted on the car to change its state of motion, no force is applied to change the state of motion of the passenger. Thus, if you are sitting on a braking car, you will keep on moving and slide out of the seat provided no net force is exerted to resist your motion due to the inertia. Seat belts are car's safety accessories that prevent passengers to follow such kind of motion of inertia and thereby protect passengers from flying out of the seat during car accidents. You could imagine how dangerous it is if you don't fasten your seat belt.
That's what may happen to a passenger with no seat belt during an accident! [ii] -
When a car accelerates, the driver would feel being pushed back to the seat due to his/her body's inertia. Besides, when a car is turning on a circular track, the driver would feel being throw away to the outside of the track. Is it also related to the inertia? Yes, it is. We should note that the direction of motion of a car would be changed if the car turns. However, according to the Newton's first law, a moving object would continue to move in a straight line provided that no net force acted on it. As a result, force must be applied to a car so that it can move along a circular track. Such force is called the centripetal force and it was shown that this force must be pointing towards the center of the circular path that the object is moved on. Thus the driver would feel being thrown outwards as the car turns. 
The centripetal force arises whenever the car turns! [iv] |
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- Newton's second law of motion
How about the case that a net force is applied to change the motion of a body moving at constant velocity (or at rest, that is velocity = 0 m/s)? What will be the resulted acceleration of this body? The answer is given by the Newton's second law. Newton's second law of motion states as follows. An object being acted by a net force will undergo an acceleration that is directly proportional to the magnitude of the force, in the same direction as the net force, and inversely proportional to its mass. Newton's second law of motion describes the behavior of objects being acted upon by a net force. This law states that the acceleration of an object depends on two variables, namely the net force acting upon the object and the mass of the object itself. Moreover, if the net force increases, the object's acceleration will also increase. Nevertheless, if the mass of the object increases, its acceleration will decrease. Thus we know that a car will accelerate at a faster rate if a greater force is applied on it. Moreover, we know that a greater force should be applied on a heavy truck so that it accelerates at the same rate as a lightweight car.  | |  | | (Courtesy: Mechanics - Team 28388) | | (Courtesy: The Physics Classroom) | 
The acceleration of an object is directly proportional to the net force applied on it. According to the second law of motion, the acceleration of an object is directly proportional to the net force acting upon the object. In other words, for a constant mass object, doubling the net force results in a doubling of the acceleration. If the net force is halved instead, the acceleration will be halved. 
The acceleration of an object is inversely proportional to the mass of the object. Newton's second law of motion also states that the acceleration of an object is inversely proportional to the mass of the object. That is to say, if the net force is held constant, the acceleration of an object will be halved by doubling the mass of the object. Similarly, "halving" the mass leads to a doubling of the acceleration if the net force is held constant again. Note that the net force imparted on an object is a quantity acting in the same direction as its acceleration. As a result, Newton's second law of motion can be written in formula as:  By making use of this formula, we can calculate the acceleration of any object being acted by a force. For example, we can find the weight of an object that tells us how large is the force of gravity acting on the object. Besides, we can calculate the braking distance of a car using the second law of motion. 
Calculating the braking distance of car by the Newton's 2nd law [v] In standard metric units, the unit of force is Newton (N). The above formula also indicates that the unit of force is given by the unit of mass multiplied by the unit of acceleration. If fact, one Newton is defined as the amount of force required for a 1-kg mass to have an acceleration of 1 m/s2. |
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- Newton's third law of motion
When you push on an object, you exert a force on it. You would expect that there is nothing else involved in this event. However, Newton predicted that there would be an equal magnitude force pushing back in an opposite direction, i.e. from the object on you. The principle behind this prediction was stated in the Newton's third law. Newton's third law of motion states as follows. For every action, there is an equal and opposite reaction. Here, the terms "action" and "reaction" refer to the action force and reaction force. In addition, the term "equal" means equal in magnitude and the term "opposite" means opposite directions. Action-reaction force must occur in pairs! 
| Newton's third law of motion tells us what will happen between two interacting objects if one of them exerts force on the other. According to this law, there would be a pair of forces called the action and reaction force acting on the two interacting objects. Moreover, the action and reaction force have equal magnitude while the two forces are acting in opposite directions, one pushing the first object by the second object and one pushing the second object by the first object. Thus Newton's third law implies that forces must occur in pairs --- equal and opposite action-reaction force pairs. You may wonder that if the action and reaction forces are really equal in magnitude but opposite in direction, why don't they cancel out with each other? In fact, this is a common misconception. The key to the answer is that action-reaction forces always act on different objects. We should note that only the forces acting on the same object could cancel each other. Therefore, forces acting on different objects don't cancel! (You could imagine it is impossible for these forces to cancel since they affect the motion of different objects.) Newton's Third Law state that an equal and opposite reaction force would arise if an object apply an action force to the other. However, we should bear in mind that these forces DO NOT CANCEL since they affect the motion of different objects.
Newton's third law of motion is closely related to the motion of a car. Assuming you are driving a car on the road, consider the motion of the car. As the wheels of the car spin backwards, they push the road backwards. In consequence, the road must push the wheels forward according to the third law of motion. It is this reaction force acting on the wheels causes the car to move forward. Note that the magnitude of the force on the road is the same as that on the wheels while the direction of the force on the road (backward) is opposite to that on the wheels (forward). Action-reaction force pairs make it possible for cars to move. The ideas of the Newton's third law of motion can be clearly illustrated by simple experiments. The so-called "rollaway players experiment", on the right, (Courtesy: Cislunar Aerospace, Inc) is one of them. Can you explain the result of this experiment?
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To Turn a Corner in a Race |
| Techniques on cornering |
 | | Two sports cars in a race |
 | | Courtesy: TurnFast: The Road Racer's Reference Center | To have a better analysis on cornering, let's define the terminology for discusson. Every corner is made of three parts, namely, the entry, the apex, and the exit. The entry is where turning begins. The apex is the point where the car reaches the furthest point on the inside of the turn. The exit is where the car is driving straight again. - The objective in driving a sports car through a corner is to have the fastest possible speed at the exit of corner. It is not necessarily to have the fastest speed going into the corner, nor even the fastest speed in the middle of the corner. The corner exit before a straight is the most important segment. The speed of the exit determines the speed during and at the end of the straight.
- In general, the fastest line through a corner is the one that allows the greatest radius, or straightest path. As a car can go faster around a large corner than it can around a tight corner, the shortest path around a corner is rarely the fastest.
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 | | Courtesy: TurnFast: The Road Racer's Reference Center | The right figure shows an illustration. The dotted line follows the path of the road. The solid line indicates a path which maximizes the radius of the turn, or attempts to make the turn as straight as possible. As you can see there is significant difference in the tightness of the turn which follows the even the outside of the road compared to one the which utilizes the whole width of the road surface. -
In addition to increase the corner radius, the path should allow the earliest possible point of getting back into the throttle. To do this, the car must be straightening back out on the corner exit path as early as possible. We can modify the above corner line further to allow this. The technique is called using a late apex. By delaying the turn-in point, and beginning the turn with a slightly sharper bend, the car can be aimed to apex later than the geometric apex point. This straightens out the second part of the turn, allowing the driver to apply the accelerator earlier. -
However, the car will have to slow down a little more at the turn in phase, but exit speed will be higher. That exit speed ensures a higher speed on the straight which will result in lower lap times overall. The solid colored line shows a path known as the "late apex." This path moves forward the point at which the car reaches the corner apex. The late apex straightens out the exit path of the car, and therefore allows the driver to apply the accelerator earlier. This increases the exit speed, and in effect lengthens the straight which allows for higher speed at the end of the straight. |
| Horsepower, work and power |
- Let us first take a look at two closely related concepts called "work" and "power". Note that the terms "work" mentioned here is not the one that we are talking about in our daily life; it refers to that defined by the physicist. Physicist stated that work is done whenever a force is exerted on an object so that it moves over a non-zero distance in the same direction as the applied force. So if the applied force is perpendicular to the direction of the resultant object's motion, no work will be done. In our everyday life, there are many examples of work done: a horse pulls a plow through the fields, a weightlifter lifts a barbell above her head, a car is crashed into a tree leading to the deformation of the car, etc. A car also does work when it is moving. Whenever a car moves, it has to apply a force to overcome the frictional forces and air resistance that resists the motion of the car. Thus work must be done in order to keep the car moving.
| (Courtesy of Quiet Valley Historical Farm) |  |  | Work must be done for either the car to move or pulling an ice block. -
The standard metric unit of work (and also energy) is the Joule (abbreviated "J"). One Joule is equivalent to one Newton of force causing an object to move along the direction of the force with a distance of 1m. -
The work done on an object can tell us how much force causes the object to move a certain distance. But it does not give us any information about how long does this force act on the object to cause the motion. In fact, we can perform the same amount of work over different time intervals. For example, a sport car takes a very short time to move from one city to another city. On the other hand, a family car takes a much longer time to complete the same journey. These two cars might do the same amount of work; however, the sport car does the work in a much shorter time than the family car. The quantity related with the rate at which a certain amount of work is done is called the power that is defined as follows:  So we can say the sport car has a greater power than the family car. -
Many machines are described by a power rating which indicates the rate at which that machine can do work upon other objects. One typical example is the car engine with the power rating usually stated in units of horsepower. The power rating of a car tells us how rapidly the car can accelerate. Suppose that a 60-horsepower engine could accelerate the car from "zero to sixty" (i.e. zero miles per hour to 60 miles per hour) in 18 seconds. If it is true, then a car with two times the horsepower could do the same amount of work in one-half of the time. That is, a 120-horsepower engine could accelerate the same car from "zero to sixty" in only 9 seconds. Some people think that it's cool to drive a car with short "zero to sixty" time and thus they want to buy a car with a higher power rating. Of course, cars with higher power rating are usually expensive as shown in the below table. Power ratings and prices of several models of cars [viii] | Model | Horsepower | Price (US$) | | Dodge Viper | 450 | $66,000 | | Ferrari 355 F1 | 375 | $134,000 | | Shelby Series 1 | 320 | $108,000 | | Lotus Esprit V8 | 350 | $83,000 | | Chevrolet Corvette | 345 | $42,000 | | Porsche Carrera | 300 | $70,000 | | Mitsubishi 3000GT bi-turbo | 320 | $45,000 | | Ford Escort | 110 | $12,000 |
- Horsepowers
 | | (Courtesy: HowStuffWorks) | - The term horsepower was invented by the engineer James Watt who is famous for his work on improving the performance of steam engines. Another unit of power called Watt was also named in memory of his work.
- The story goes back to 1783 when Watt wanted to find out how much power is available from a pony to lift coal at a coal mine. He found that, on average, a mine pony could do 22,000 foot-pounds of work in a minute. Therefore Watt judged that one horse could do 33,000 foot-pounds of work every minute. So he defined one horsepower to be 550 pound-feet per second, i.e. 33,000 foot-pounds per minute. The horsepower can be converted into units of Watt where one horsepower is found to be equivalent to 746 Watts.
- Horsepower is an arbitrary unit of measure of power that has been used for over centuries. Nowadays, it is still widely used and appears on cars, air conditioners and even on some vacuum cleaners!
- The standard metric unit of power is the Watt (abbreviated "W"). From the formula of power, a unit of power should be equal to a unit of work divided by a unit of time. A Watt is equivalent to the Joule (unit of work) per second (unit of time). Horsepower is another commonly used unit of power where one horsepower is equivalent to about 750 Watts.
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Have you ever noticed any car advertisement with specifications like "this engine makes 267 pound-feet of torque at 4,300 rpm"? In fact, you will find that the torque of a car is often mentioned in many car advertisements. What does the term "torque" means? How is it related with the quality of a car?
Combustion Engine of a Car |
2003 Jeep Grand Cherokee Engine [viii]  |
Engine is one of the most important parts of a car since it provides the power for the motion of the car. The working of the car engine plays a crucial factor in the functioning of the car.
What happens inside a car engine? The car engine is the place in which the fuel gasoline is converted into the power of a car. Therefore, a car engine is an internal combustion engine since combustion takes place internally. Why the external combustion engine is not used as car engine? It is because internal combustion is much more efficient (takes less fuel per mile) than external combustion.
| How Things Work |
- Internal combustion
A good example of internal combustion is the war cannon. A cannon is lighted after the soldiers load it with gunpowder and a cannon ball. And that is internal combustion. The "firing" of the potato cannon [viii] 
| The internal combustion taking place inside the car engine is very powerful. To let you understand how powerful it is, let us look at a more relevant example --- a device commonly known as potato cannon. It consists of a big piece of plastic sewer pipe, say 3 inches in diameter and 3 feet long, with a cap on one end of it. A tiny drop of gasoline is put into the cannon and a potato is stuffed down the pipe. Then the fuel can be ignited by introducing a spark. After the ignition, you will see that the potato would be launched about 500 feet through the air (just like in the above figure)! There is a huge amount of energy in a tiny drop of gasoline. The basic principle behind the potato cannon is the same as that for any internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small confined space and ignite it, a huge amount of energy would be released in the form of expanding gas. In the potato cannon, this energy is "translated" into the potato motion and thus the potato can be propelled 500 feet. Besides, if you can create a cycle that allows explosion like this occur hundreds of times per minute and then the energy is harnessed in a useful way, you can get a very huge amount power from this cycle. In fact, this is what happens inside the core of a car engine! Today, most of the car engines are the so-called four-stroke compression engines in which the four-stroke combustion cycle is used to convert gasoline into motion. The four-stroke combustion cycle, also named Otto cycle, is invented by the Germans Nikolaus Otto in 1867. The "four-stroke" refers to the four-times movement of the piston over one revolution of the engine in each cycle, namely, intake stroke, compression stroke, combustion stroke and exhaust stroke. - What's inside a car engine?
The internal structure of a four-stroke compression car engine [viii] Before studying the details of the four-stroke combustion cycle, let us first take a look on the internal structure of the car engine. The above figure shows a four-stroke compression engine with only one cylinder. In each cylinder, there is a piston (will be discussed in details later) and a confined space called the combustion chamber in which combustion and compression of fuel occurs. At the top of the cylinder, the intake valve allows the air and fuel mixture to enter the combustion chamber while the exhaust valve allows the exhaust gas produced by the combustion to leave the chamber. Note that both valves are closed during compression and combustion of the fuel in order to seal the combustion chamber. To ignite the air and fuel mixture, the spark plug at the top of the cylinder supplies the spark so that the combustion can occur. Of course, the spark must happen at just the right moment for the engine to work properly. Just beneath the valves, a cylindrical piece of metal called the piston moves up and down inside the cylinder. Indeed, the piston plays the same role as the potato in the potato cannon, i.e. it will be "fired" by the explosion of the fuel. The connecting rod connects the piston to the crankshaft which converts the piston's up and down motion (linear motion) into rotational motion just like a crank on a jack-in-the-box does. The rotational speed of the crankshaft is indeed the engine speed that people talking about. Since the angle of the connecting rod will be changed as the piston moves and the crankshaft rotates, the connecting rod is made to be capable of rotating at both ends. At the bottom of the cylinder, the crankshaft is surrounded by the oil sump containing of some oil being collected in the sump's bottom (the oil pan). The use of the oil is to lubricate the moving parts of the engine like the piston. To prevent the fuel/air mixture and exhaust gas leaking into the sump during compression and combustion, the piston rings are installed between the outer edge of the piston and the inner edge of the cylinder which provides a sliding seal between them. The piston rings can also prevent oil in the sump from leaking into the combustion area, where it would be burnt and lost. - The four-stroke combustion cycle
Let us come back to the discussion about the four-stroke combustion cycle. The movie below illustrates how the four-stroke combustion cycle works in an engine with only one cylinder. When the engine goes through one cycle, the following happens: The four-strokes combustion cycle 
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| 1. | The first stroke is the intake stroke. In this stroke, the intake valve opens as the piston moves down from the top. As a result, a cylinder-full of air and gasoline is taken into the engine. For the combustion to occur properly, the air needs to be mixed with the tiniest drop of gasoline. (Part 1 of the animation) | |
| 2. | Then the piston moves back up to compress this fuel/air mixture between the top of the piston and the top of the cylinder. Since the compression raises both the pressure and temperature of the mixture, the explosion becomes more powerful. This stroke is known as the compression stroke. (Part 2 of the animation) |
| 3. | As the piston approaches the top of the cylinder, the spark plug emits a spark to ignite the fuel and air mixture. The resulting explosion produces a lot of exhaust gas leading to a rapid increase in the pressure inside the cylinder. Subsequently, the piston is forced to move back downwards. During this so-called power stroke, power is transmitted from the piston to the crankshaft, which will be in turns transferred to the wheels through the other parts of the car. (Part 3 of the animation) |
| 4. | In the last stroke, the exhaust valve opens and the exhaust gas leaves the cylinder after the piston hitting the bottom of the cylinder. Thus this stroke is also known as the exhaust stroke. (Part 4 of the animation) |
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After completing the four strokes, the piston reaches the top of the cylinder again and the condition of the cylinder is restored to be the same as that just before the intake stroke. So now the engine is ready for the next cycle and thus the engine can repeat the cycle again and again.
Different types of multi-cylinder engine The engine discussed above has only one cylinder. However, most cars engine has more than one cylinder like four, six or eight cylinders. We should aware that the cylinders inside the multi-cylinder engines do not fire at the same moment. For example, if a cylinder is in the stroke one, the other cylinders may be in the stroke two and so on. The exact sequence of the firing of the cylinders is known as the firing order. In a multi-cylinder engine, the cylinders can be arranged in different orders. The typical arrangements of the cylinders are: inline, V or flat (also known as horizontally opposed or boxer). The following animations show how these multi-cylinder engines work.
A flat four-cylinder car engine [viii] (click the picture for animation)
An inline four-cylinder car engine [viii] (click the picture for animation)
A V-6 car engine [viii] (click the picture for animation)
Different configurations (inline, V or flat) have different pros and cons in smoothness, manufacturing-cost and shape characteristics. They are chosen according to the requirements of different vehicles.
Insight - Displacement of a car engine
During the combustion cycle, the piston moves up and down which changes the size of the combustion chamber from its maximum volume to the minimum volume. The difference between the maximum and minimum volume is known as displacement. The displacement is usually measured in liters or CCs (Cubic Centimeters, where 1,000 cubic centimeters equals one liter). For example, a motorcycle might have a 500 cc or a 750 cc engine while a sports car might have a 5,000 cc engine.
When people mention the displacement of a car engine, they are indeed referring to its total displacement that is given by the sum of the displacements of all the cylinders inside the engine. Thus a 4-cylinder engine in which each cylinder displaces half a liter is called a "2.0 liter engine". On the other hand, if six half-liter displacement cylinders are arranged in a V configuration inside the engine, the engine is called a "3.0 liter V-6 engine".
In general, the displacement gives you an idea about how much power an engine can produce. A 500 cc cylinder can hold twice the amount of the fuel and air mixture holding by a 250 cc cylinder. Therefore we would expect the 500 cc cylinder produces about twice as much power as the 250 cc cylinder provided that everything else is equal. In other words, the engine with larger displacement is considered to be more powerful. Most modern cars have engine with displacement of ranged from 2000 cc to 5,000 cc while trucks often have displacements much higher than 5000 cc. The following table lists the displacements of the engines of several vehicles.
Displacement of the engines of several vehicles [iii] | Model | Displacement (cc) | Model | Displacement (cc) |
| Chevy Camaro SS | 5665 | Ford SVT Mustang Cobra | 4601 |
| Audi A6 | 4172 | BMW S40i | 4391 |
| Lexus GS 430 | 4293 | Dodge Stratus ES | 2736 |
| Honda Accord EX | 3000 | Chevy 2500 HD 2WD (truck) | 8000 |
| What could be wrong inside the car engine? It is not rare that a car suddenly breaks down on a road. Such circumstance is probably due to something wrong inside the car engine. What could be wrong inside the engine? |
| Explanations: The malfunction of the engine are mainly due to the following reasons: - Bad fuel mix - If bad fuel mix occurs, the combustion cannot carry out properly and thus the engine cannot run in the proper way. The bad fuel mix may be due to the shortage of fuel in the air and fuel mixture after running out of gasoline. If there is clogging of the air intake, there would be insufficient air in the air and fuel mixture so that it can be burn properly. Besides, the bad fuel mixture might be the product of either inappropriate amount of fuel supplied by the fuel system or existence of impurity (like water in your gas tank) in the fuel.
- Lack of compression - If the air and fuel mixture is not compressed properly, the combustion process will not work in the right way. The lack of compression might be due to the worn out of piston rings that allows the air and fuel mixture to leak past the piston during compression. If the intake or exhaust valves are not sealed properly, leaking of fuel mixture might also occurs during compression. In addition, the leaking may be due to the presence of a hole in the cylinder. The "hole" is commonly found to be located at the position where the top of the cylinder attaches to the cylinder itself.
- Lack of spark - If there is only weak spark or even no spark for the ignition of fuel, the combustion process will also work improperly. The lack of spark must be related with spark plug. For instance, if spark plug or the wire leading to it is worn out, the spark will be weak. More severely, there will be no spark if the wire is cut or missing, or if the system sending the spark down the wire is not working properly. On the other hand, problem about the fuel ignition will also arise if the spark occurs either too early or too late during the combustion cycle that makes the fuel does not ignite at the right moment.
Of course, there are many other causes for the engine to be out of order. For example, it may be resulted from out of power inside the battery so that the engine cannot be started. Another possible cause for the malfunction of the engine may be the running out of oil inside the cylinder and thus the piston cannot move up and down freely. |
Suppose you are driving your car on a road. Suddenly, a boy rushes out to the road in front of your car. To avoid hitting the boy, you press the brake petal to stop the car. Whether you can avoid hitting the boy or not depend mainly on the quality of you car's brake system. This story illustrates that a good brake system are vital for avoiding car accidents. In this section, let us take a close look on the brake system, which will cover the basic scientific principle behind the system, the workings of a simple brake system and the antilock braking system (ABS).
A typical brake system [viii]
| How Things Work |
- Force of friction
When the brake is pressed, a huge stopping force would be generated. Obviously, this force would stop the rotation of the wheels in order to stop the motion of the vehicle. How can this be done? In fact, the stopping force slows down the spin of the wheels by generating friction on it. To understand the working mechanism of the braking system, let us begin by taking a quick look on friction. When one surface slides over another surface, the so-called frictional force that resists the sliding motion would appears. Friction is a measure of how hard it is to slide one object over another (i.e. for the sliding between the surface of the two objects). It was found that the friction depends on the roughness of the contacting surfaces. Indeed, friction would exists even the surfaces are very smooth. | Why friction appears? | The friction is due to the irregularities on a surface. If we look at any surface through the microscope, you will see many small peaks and valleys (the irregularities). If two surfaces rub over each other, the irregularities on the two surfaces would get squished together resulting in a frictional force that opposes the motion. We should note that frictional force exists even the surfaces in contact are not in relative motion. For example, if you push a heavy block slightly, the block might not move. It is because you have not applied a large enough force to overcome the frictional force between the block and the floor under the block. |  | | Friction at microscopic scale [viii] | | Suppose you want to push two different weight blocks to another location by the bulldozer as shown in the figure below. If the two blocks are made of the same material, which one will be harder to push? In fact, the heavier one will be harder for the bulldozer to push. Why? It is because the weight of the heavier block causes the irregularities on the contacting surfaces to squish together much more and thus gives rise to a larger frictional force for the heavier block. It implies that the amount of force required to move a block is proportional to that block's weight; larger force is need for heavier weight. This principle has been applied for many devices such as brakes and clutches, where a pad is pressed against a spinning disc. More force pressing on the pad would result in greater stopping force. |  | | Which block has a larger friction? [viii] | | In addition, the friction resisting the slide of a block would also depends on the material made up the block since different materials have different microscopic structures. For example, sliding rubber against rubber is harder than sliding steel against steel. | The friction force of sliding a block over a surface is measured by the coefficient of friction that is the ratio of the force required to slide the block to the block's weight. The coefficient of friction is a dimensionless unit and has a value greater than or equal zero. A smaller value means a more slippery surface. For a perfectly smooth surface, the coefficient of friction is zero. To be concrete, let us consider an example. Suppose the coefficient of friction for sliding blocks made of certain material over a surface is 1.0, then it would take 100 pounds of force to slide the 100-pound block, or 300 pounds of force to slide the 300-pound block. If the coefficient of friction is 0.1 rather than 1.0, then it will take 10 pounds of force to slide to the 100-pound block or 30 pounds of force to slide the 300-pound block. We should be aware that the coefficient of friction would be different if the surfaces of contact are relatively in motion instead of at rest. The coefficient of static friction refers to the case that the two surfaces in contact are not sliding relative to each other. Moreover, the coefficient of dynamic friction corresponds to the case that the two surfaces are sliding relative to each other. The coefficient of dynamic friction is usually less than the coefficient of static friction, which implies more force is required to move a static object than to keep it moving. The following table lists the coefficients of friction for a few instances. The coefficients of friction for several cases [iii] | Coefficient of
static friction | Coefficient of
dynamic friction | | Rubber on concrete | 0.90 | 0.70 | | Copper on glass | 0.68 | 0.53 | | Oak wood on oak wood | 0.54 | 0.32 | | Steel on ice | 0.02 | 0.01 | The table below shows that the coefficient of friction of the tires of a vehicle moving on a concrete road varies a large amount under different circumstances. The coefficients of friction of the vehicle's tires under different circumstances [iii] | Coefficient of
static friction | Coefficient of
dynamic friction | | Dry concrete at low speed | 0.9 | 0.7 | | Dry concrete at high speed | 0.6 | 0.4 | | Wet concrete at low speed | 0.7 | 0.5 | Besides, the coefficient of friction is also related with the stopping distance that is the shortest distance for a vehicle to stop without skidding. In particular, it was found that the stopping distance is inversely proportional to the coefficient of static friction for the tires to slide over the road. As mentioned before, the coefficient of static friction of the tires changes a lot if the tires becomes worn out or the road condition changes due to the weather (see below table). Therefore, the stopping distance of the vehicle varies a large amount under different circumstances, as shown in the below table. For example, the stopping distance of a vehicle with new tires moving at 60 mph increases eighteen times if the vehicle moves from a dry road to an icy road. In brief, both the road condition and condition of the tires are crucial factors determine whether a vehicle can be easily stopped on a road. The coefficients of static friction under various conditions for new and worn tires [iii] Condition
of tires | Weather | | Dry | Wet
(light rain) | Heavy rain
(puddles) | Ice | | At 60 mph | | New | 0.9 | 0.60 | 0.3 | 0.050 | | Worn | 0.9 | 0.20 | 0.1 | 0.005 | | At 80 mph | | New | 0.8 | 0.55 | 0.2 | 0.005 | | Worn | 0.8 | 0.20 | 0.1 | 0.001 | The stopping distance of a vehicle moving at 60 mph under different conditions | Condition | Stopping distance (feet) | | Dry (new tires) | 134.4 | | Light rain (new tires) | 201.6 | | Light rain (worn tires) | 605 | Heavy rain
(puddles and worn tires) | 1210 | | Ice (new tires) | 2420 |  | | The tire pattern (Courtesy: The Wikipedia) | - The tread of a tire
In order to increase the traction or the skid resistance between the tires and the road surface, the tread pattern is introduced to the tire surface. Under dry conditions on paved roads, a smooth tire gives better traction than a grooved or patterned tread because a larger area of contact is available to develop the frictional forces. For this reason, the tires used for auto racing on the tracks have a smooth surface with no tread design. Unfortunately, a smooth tire develops very little traction under wet conditions because the frictional mechanism is reduced by a lubricating film of water between the tire and the road. A patterned tire provides grooves or channels into which the water can squeeze as the tire rolls along the road, thus again providing a region of direct contact between tire and road. A patterned tire gives typical dry and wet frictional coefficients of about 0.7 and 0.4, respectively. These values represent a compromise between the extreme values of about 0.9 (dry) and 0.1 (wet) obtained with a smooth tire. In a rainy day, when there is too much water on the road cars will hydroplane if they are going too fast. Hence, the water gets between the tires and the road leaving an unexistent frictional force. The tires are not in contact with the road so the frictional force is gone causing the car to slid until the tires make contact with something that will cause there to be a frictional force. Classical friction theory in the high school does not work well to the case of rolling wheels. The reason is that the tires have structural flexibility and tread rubber stretches when they are rolling on the road. Instead of depending solely on the coefficient of friction at the tire-road interface (which is determined by the nature of the road surface and the tread rubber compound), maximum stopping ability also depends on the resistance of the tread to tearing under the forces that occur during braking. When a car is braked to a hard stop on a dry road, the maximum frictional force developed can be greater than one's initial thought. The result is that instead of the tire merely sliding along the road, rubber is torn off the tread at the tire-road interface. Undoubtedly the tread resistance to this tearing is a combination of the rubber strength and the grooves and slots that make up the tread design. The heat energy generated due to tearing of rubber reduces the speed of the car.
Some very high performance cars have both directional and asymmetric tires, but they have to be fitted the right way around on the wheels and on the correct side of the vehicle. Also, the performance of the tires varies and it depends on the rubber compounds. Generally, the softer the compound the better the grip on the road, but as a result they usually wear a lot quicker than harder compound tires. - Basic principle of the brake system
When the brake pedal is pushed, the car transmits the force from driver's foot to its brakes through some fluid. Obviously, actual brakes require a much greater force than that exerted by driver's leg. Thus the car must magnify the force of driver's foot. In the brake system, the multiplication of force is attained by making use of the two principles: | 1. | Leverage | | 2. | Hydraulic force multiplication | Let us study these two principles one by one. - Leverage
Before any force is transmitted to the brake fluid, the pedal multiplies the force from driver's leg several times by using a lever. The force multiplication gained by the use of lever is called the leverage. The lever is based on the principle of lever stating that: "The force pushing upward at one end of a lever is equal to the force pushing downward at the other end times the ratio of the distance of the two forces from the pivot point. The below figure illustrates the principle of the lever. 
The principle of lever [xii] So we can get a bigger force at one end of a lever by applying a force at a suitable position on the other end. Let us consider an example to explain how to do this. In the figure below, suppose a force F is being applied to the left end of the lever which is twice as long (2X) as the right end (X). Then a force of 2F would be available on the right end of the lever. Besides, the force at the right end should act through half of the distance (Y) that the left end moves (2Y). If we change the relative lengths of the left and right ends, the force multiplication by the lever (i.e. the ratio of the two forces) changes. 
Force multiplication by a simple leverage [viii] - Hydraulic force multiplication
After multiplied by the lever, the force acted on the pedals would be magnified by a hydraulic system consisting of some cylinders with brake fluid inside. In fact, the workings of any hydraulic system is based on the so-called Pascal's principle: The pressure exerted on a confined incompressible fluid is transmitted undiminished in all directions and acts with equal force on all equal areas (i.e. pressure is constant). where the pressure is the force divided by the area.  A simple hydraulic system [viii] How does a hydraulic system work in practice? Let us consider a simple hydraulic system. In the above figure, two pistons are fit into two oil-filled cylinders which are connected together with an oil-filled pipe. If a downward force is pushed on one piston (the left one, in this drawing), then the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, almost all the applied force arrives at the second piston. (Of course, there would be some loss due to friction or other reasons.) The advantage for using the hydraulic systems is that the pipe connecting the two cylinders can be any length and shape. So the connecting pipe can be a fine irregular shape pipe so that it can snake through all sorts of things separating the two pistons. The pipe can also have more than one openings at each end and thus one master cylinder (the cylinder where the force is applied) can drive more than one slave cylinder (the cylinder where the force is transmitted to) just like that in the below figure.  Master cylinder with two slaves [viii] Another advantage for using a hydraulic system is that the force multiplication (or division) can be performed easily. According to the Pascal's principle, the pressure applied at one end of a hydraulic system would be transmitted with constant value throughout the system. Therefore, to change the force on a piston in a hydraulic system, all we have to do is change the size of one piston, as shown below.  Force multiplication by a hydraulic system [viii] We can determine the factor of the force multiplication in the above system by looking at the size of the pistons. Suppose the radius of the piston on the left is 1 inches while the radius of the piston on the right is 3 inches. For a piston (which is in circle shape), the area is equal to Pi*radius*radius. So the right piston must be nine times larger than the left piston as the radius of the right piston is three times larger than that of the left piston. Hence, any force applied to the left-hand piston will increase nine times as it come out on the right-hand piston. That is to say, if a 100-pounds downward force is applied at the left piston, a 900-pound upward force will arise on the right. However, the right piston will only raise 1 inch if the left piston is depressed by 9 inches since the volume of the fluid displaced by the two pistons must be the same. - A simple braking system After learning the basic principle of the brake system, let us look at a simple brake system as shown below.
 A simple brake system [viii] In this system, the distance from the pedal to the pivot is four times larger than that from the cylinder to the pivot, so the force applied at the pedal will be increased by a factor of four before it is transmitted to the cylinder. Then the hydraulic system magnified the force by nine times since the ratio of the radius of the brake cylinder to that of the pedal cylinder is three. Thus this system all together increases the force of the driver's leg by a factor of 36. For example, if a 10-pound force is pushed on the pedal, a 360-pound force will be generated to squeeze the brake pads at the wheel. There are a number of possible problems for this simple system. For instance, what will happens if there is a leak in the system? If there is only a slow leak, eventually the brake cylinder will not have enough fluid for the brakes to work. In contrast, if there is a major leak, then all the fluid will rush out the leak at the first time the brakes is stepped; and the result is a complete brake failure. To avoid accident due to brake failure, the master cylinder in modern cars is designed to handle these potential problems. - Antilock braking system (ABS)
The antilock braking system (ABS) is designed to avoid the car went out of control when the brakes lock up. Thus the system can help the driver to stop his/her car safely even on a very slippery surface. Obviously, a skidding wheel has less traction than a non-skidding wheel. By keeping the wheels from skidding while the driver slow down, anti-lock brakes benefit him in two ways: He'll stop faster, and he'll be able to steer while he stops. The ABS system has been used on cars since the late 1960s. Nowadays, almost all new cars are equipped with this system. However, the ABS does not always give rise to a shorter stop for any surfaces. For example, it takes almost the same time for a car to stop on a dry concrete road either by the regular brake system or ABS. However, the ABS would allow the car to stop faster on a wet or icy road. The main function of the ABS is to avoid the driver losing control of the car. 
The components of the antilock brake system (ABS) [viii] ABS works by monitoring the speed of all the wheels of the car all the time. Based on the data received from the speed sensors mounted on each wheel, the ABS controller looks for any abnormal decelerations in the wheel. It is because a wheel experiences a rapid deceleration just before it locks up. It might take five seconds for a 60 mph car to stop under ideal conditions; however, a wheel that locks up could stop rotation in less than one second. In other words, if the deceleration were not intervened, the wheel would stop at an abnormally fast rate. Knowing that the ABS does not bear such a rapid deceleration, the ABS controller reduces the hydraulic pressure to that brake until the wheel accelerates again. Then it restores the hydraulic pressure until the abnormal deceleration appears again. It can do this so fast that there is no significant change in the speed of the tire. Due to the functioning of the ABS, the tire would slow down at the same rate as the car with the brakes keeping the tires to undergo maximum deceleration for no skidding. It results in maximum braking power for the system. Due to the rapid stopping and starting of the hydraulic pressure by the ABS, the person stepping the brakes will feel a pulsing of the brake pedal when the ABS operates. Such pulsing can occur to 15 times per second for some ABS systems. The ABS system is a valuable component of modern cars that improves the safety of driving. However, the ABS cannot avoid all skids. For example, the ABS cannot prevent those skids caused by excess speed, sharp turning and slamming on the brakes. In fact, many skidding occurs before the ABS is activated. However, ABS significantly reduces the stopping distance and helps you to keep control of your car. |
In the modern world, road's traffic is busy and traffic accidents happen frequently. In each year, many passengers and drivers are injured or even killed by traffic accidents. In fact, there are many measures that can be taken by drivers or passengers to reduce the chance of getting injured or killed by car accident.
| How Things Work |
- Crashworthiness
A severely crashed car [ix] One of the approaches to improve driving safety is reducing the degree of injuries in case of car crash. Crashworthiness is the measure of how well a vehicle protects its occupants during a crash. Therefore, the crashworthiness of a vehicle give us an estimation about how bad the passenger would be hurt if this vehicle crashes with other object like a tree or another vehicle. Since high crashworthiness refers to less expected degree of injuries, it should be more safe for us to ride on a vehicle with higher crashworthiness. In the study of the crashworthiness, researchers investigate the cause and mechanism of the injury of the driver and passenger during a collision. The results of this investigation also allow researchers to develop some means for estimating the severity of a collision. Based on the knowledge gained from the study of the crashworthiness, we can get some clues about whether anything could be done to reduce the chance and severity of human injuries during the car crash. We should be aware that crashworthiness is not the same as vehicle safety. When we consider the crashworthiness, we have assumed that the car crash has already happened. Unlike in the study of vehicle safety, we don't care about who is responsible for the accident or whether the accident could be avoided in the study of the crashworthiness. Therefore, it is possible that a relatively safe vehicle has poor crashworthiness. That is to say, even if a car is equipped with many crash avoidance features, it might have features causing severe injuries in case that the car crash really happens. - Crash test
As mentioned above, if we want to know the risk of injury of the occupant of a vehicle during an accident, we can take a look on the crashworthiness of this vehicle. But how do people determine the crashworthiness of a vehicle? It is determined by performing crash test for the vehicle. Crash test dummies play an integral part of vehicle crash tests. The use of the dummy is to simulate a human being during a crash and also collect data that cannot be collected from a human occupant. In the United States, all frontal crash tests are conducted using the same family of dummy called the Hybrid III dummy in order to guarantee consistent results. 
The family of the Hybrid III dummy [x] | | 
A Hybrid III 50th Percentile Male dummy [x] | During the car crash, the degree of injuries might be different for passengers of different gender and age even they are under the same conditions. So dummies having different dimension and internal structure are used in the crash test of a vehicle. Dummies are classified by the percentile and gender. For example, the 50th percentile male dummy represents the median sized male, i.e. it is bigger than half the male population and smaller than the other half. This dummy weighs 170 lbs and is 5 ft 10 inches tall. It is the dummy most widely used in frontal crash test all over the world. | Crash test dummies | To make them human-like, all crash test dummies are built of materials that simulates the physiology of the human body. For example, the upper part of the adult Hybrid III dummy is made of several metal rods with polymer based damping material which simulates the reaction of the chest of an adult during the collision of cars.
On each dummy, there are three types of instruments: | Accelerometers - These devices measure the acceleration in a particular direction. The acceleration or deceleration (i.e. the rate of decrease of the speed) of the body during the crash is a crucial factor determining the probability of injury. For example, if you crash your head into a brick wall, the deceleration of your head is very large and so you are probably hurt. In contrast, if you crash your head into a pillow, the deceleration of your head is much smaller and so it doesn't hurt. To measure the acceleration of different parts of the body, accelerometers are installed all over the dummy like dummy's head, chest, pelvis, legs and feet. | | Load sensors - These sensors measure the amount of force acting on different parts of the body during a crash. If we find the maximum load in a bone, we can determine the probability of breaking of this bone. | | Motion sensors - These sensors are installed in the dummy's chest to measure the deflection of the occupant's chest during a crash. If the deflection of the chest is large, it is likely that the occupant is severely injured during the crash. | Before the crash-test, dummies are placed in the vehicle after researchers has applied paint to them. Different colors of paint are applied to the parts of the dummies' bodies including knees, face and skull which are most likely to be stroked during a crash. The paint can help us to locate where the different parts of the body hit the car. For example, in the following photo, the blue paint sticked on the airbag indicates that it was hit by the dummy's face. |  | | The multicolored paint on the dummy shows the position of the cabin hit by different body parts. [x] | | If the accelerometers in the dummy's head indicate there is a very large acceleration, it means that the risk of injury is very large during the crash. Researchers can look at the paint mark to find out which body part hit what part of the vehicle inside the cabin, helping them to develop improvements to prevent such injury in future crashes. | The frontal crash tests and the side impact crash tests are the major types of crash test used to determine the crashworthiness of a vehicle. In the frontal crash test, a vehicle moving at a fixed speed strikes straight with a rigid barrier. It is equivalent to the head-on collision between two vehicles having the same weight and same speed as the test vehicle. For frontal crash tests, crash-test dummies are placed in driver and front passenger seats with seat belts fastened. The frontal crash tests can be subdivided into two types --- the full-width frontal crash test and offset frontal crash test. The main difference is that the full width of a vehicle's front is stroked in the full-width frontal crash test while only one side of the vehicle's front is stroked in the offset frontal crash test. The results from these two tests complement each other and can be used together to assess overall frontal crash safety. | (Courtesy: NHTSA) |  | | (Courtesy: IIHS) | |  | |  | | A full-width frontal crash test demo | | A offset frontal crash test demo | In the side impact crash tests, an impactor with a deformable front end representing the front of a car crashes into the side of the test vehicle. It simulates a car that is crossing an intersection being hit its side by a car running a red light. For side crash tests, dummies are placed in the driver and rear passenger seats at driver's side; and they are both secured with the vehicle's seat belts. We should note that test results of any crash test should be compared only among similar weight vehicles. | (Courtesy: IIHS) | |  | | A side impact crash test demo | In the United States, the National Highway Traffic Safety Administration (NHTSA) and the Insurance Institute for Highway Safety (IIHS) conduct crash tests for a large number of different models of vehicles. Based on the results of their crash test, these two organizations set up two systems of safety ratings for the crash of vehicles. NHTSA rates the safety of crash of vehicles by stars with five stars meaning the smallest risk of injury in the crash. On the other hand, vehicles are rated as good, acceptable, marginal and poor by the safety ratings of IIHS. The crash ratings of vehicles can be found on the webpages of NHTSA and IIHS as well as many publications. The table below shows the crash safety ratings for several 2002 vehicles. Crash safety ratings for several 2002 vehicles [iii] | Model | NHTSA
(Driver/passenger) | IIHS | | Audi A6 | ****/**** | Acceptable | | Chevy Impala | *****/***** | Good | | Infiniti QX4 | ****/***** | Marginal | | Lincoln LS | *****/***** | Good | | Ford Taurus | *****/***** | Good | | Plymouth Neon | ****/**** | Marginal | Of course, the best way to reduce the chance of getting injured during a crash is to avoid the crash. The ability of crash avoidance depends on many factors such as the presence of ABS, types of types, how sharp the car can turn and so on. But perhaps the most important factor is whether the driver has good ability and skill or not. - Collision protection devices
The safety system of a car [xi] (Click the picture for full size) Most modern cars are equipped with a safety system consisting of many collision protection devices. The seat belt is one of the most important protection devices which have saved thousands of lives in car accidents. By tiding the person with the car, the seat belt makes the person to slow down at the same rate as the car and not be thrown out of the car. To give more protection to the passenger, the air bags are usually used in conjunction with the seat belts. It was found that injuries could be significantly reduced by using air bags with the seat belts. For example, statistics show that air bags reduce the risk of dying in a head-on crash by 30 percent. When car crashes, the air bag is quickly inflated with a large volume of gas and then bursts out to the space just in front of the passenger. Then the passenger would hit on the cushion-like air bag and thus slow down his/her speed over a much longer time. As a result, the amount of force acting on the passenger is reduced. But we should note that it would be dangerous to use the air bags under specific conditions. For example, it was revealed that the air bag could hurt someone who is too close to it. So a passenger might probably injured by the air bag if he/she does not wear a seat belt. Another important collision protection device is the crumple zone in the front and rear of the car. A good crumple zone can allow the car to have a longer time of impact and thereby decrease the magnitude of the force acting on it. Other safety features such as collapsible steering wheels, side air bags and headrest are also critical for the protection of passengers during a collision. - Seatbelt
Seatbelt [viii] 
| When a car is moving, it has its inertia. The riders on it also own inertia. The rider can be stopped if there is a force acting on it. However, the force should be acted on more durable parts of the body over a longer period of time in order to reduce the chances of major injury. The basic idea of a seatbelt is very simple: It keeps you from flying through the windshield or hurdling toward the dashboard when your car comes to an abrupt stop. A seatbelt spreads the stopping force across sturdier parts of your body. A seatbelt consists of a lap belt, which rests over your pelvis, and a shoulder belt, which extends across your chest. The two belt sections are tightly secured to the frame of the car. When the car stops abruptly, the stopping force will apply to a larger area e.g. the rib cage and the pelvis, and cause less demage. Moreover, the seatbelt webbing is made of more flexible material, it stretches slightly but not more, otherwise you might bang into the steeing wheel or side window. A seatbelt will only let you shift forward slightly and hold you in place. The extended time duration for you to stop your body decreases the force acting on you and thus a better protection on your life. |
The urge for the preservation of natural resources and a reduction to the pollution of the environment has led car manufacturers to seek alternative engines. Other than the standard gasoline and diesel combustion engines, there are gas turbine, battery and solar-powered engines, gasoline-electric hybrids and fuel cells, etc.
| Electric cars |
An electric car [viii] 
| To recharge a car [viii] 
| |
- The downsides of electric cars are their inefficiency and the need for heavy batteries.
- The cars have short operating range, low speeds and long charging periods make the cars impracticable for many applications.
- The upsides are the cars producing no harmful emissions, the pollution is created indirectly by the power stations that generate the electricity to charge their batteries.
- They are applicable to local delivery vans and forklifts, where speed and range are unimportant.
|
Solar-powered cars A Solar Car; courtesy: The Wikipedia 
| - These cars are "pollution-free" to the environment and are cheap to run.
- However, each photovoltaic (solar) cells or the chemical batteries produce very small amount of electricity.
- A solar-powered car needs hundreds of cells to generate enough power to move, and thus a rather high initial cost.
- The situation will be improved after technical improvement of the solar cells and the lowering of the cost of them.
|
Hybrid cars - These cars use dual fuel gasoline-electric hybrid. The gasoline engine drives the car and at the same time it recharges the batteries through the generator. These cars use the gasoline engine for traveling outside town and the electric engine in local environments. It provides flexibility for applications.
- It applies the regeneratives braking system and recharge the batteries while slowing down the car.
|
Fuel-cell-powered cars - The fuel cell of this vehicle makes use of hydrogen to react with atmospheric oxygen. It produces electricity, water and heat. The by-product is clean and there is no emission of hazardous greenhouse gases, e.g. carbon dioxide. However, hydrogen is highly flammable and the storage of it should be safe, so the cell applies methanol as the hydrogen source despite of the emission of some pollutants (far fewer than the combustion engines) when the methanol generates hydrogen.
- The efficient of fuel cell is high and it is durable.
|
| Supplementary notes on the laws of motion |
(Courtesy: The Physics Classroom)  | - More examples on Newton's first law of motion
In our daily life, there are many phenomena that could be explained by the Newton's first law of motion. Some of them are listed as follows: - Blood rushes from your head to your feet when riding on an elevator that suddenly stops during downward motion.
- The head of a hammer can be tightened onto the wooden handle by hitting the bottom of the handle against a hard surface (see the picture on right hand side).
- Headrests are placed in cars to prevent neck injuries due to the rapid back and forth motion of the head during car accident.
- During a ride on a skateboard, if the motion of the skateboard suddenly halts after hitting an obstacle like a rock, you will fly forward off the board.
- Some illustrations on movies
Below are some simple experiments illustrating the first law of motion. | I. | Penny-in-a-glass (Courtesy: Cislunar Aerospace, Inc) | | II. | Smash Your Hand (Courtesy: The University of Minnesota) | | III. | Car on Cart on Cart (Courtesy: The University of Minnesota) | - Try to explain the following observation.
| Observation: An empty bottle lies on the floor of a train compartment which is at rest. When the train starts moving, the bottle rolls back; while the bottle rolls forward when the train brakes to stop. Why this happens? | | Explanation: When the train starts, the bottle tends to stay in the state of rest due to its inertia. So it rolls back to the original position as the train starts to move. Similarly, when the train stops, the bottle tends to continue its motion due to its inertia and thus it rolls towards the direction the train was headed. The bottle stays at rest only when the train is moving with constant velocity. | 
An empty bottle on the floor of a train while it starts and stops. | | |
- Mass and weight
| Does mass have the same meaning as weight? | | According to Newton's second law of motion, the behavior of an object depends on its mass. A more massive object has more inertia and is more difficult to be accelerated. So it seems that mass has the same meaning as weight. But this is WRONG!! Indeed, weight and mass are related by:  Note that weight is a force. The acceleration of gravity is the acceleration of any body undergoing free-fall with the negligence of the air resistance. It is roughly constant at anywhere of the earth's surface with value equal to one "g", i.e. 9.8 m/s2. So we may interpret that weight is the force of gravity acting on a body. |  | | On the Moon, a person's weight is only 1/6 of his or her weight on the Earth. [vi] | | Obviously, different planets have different force of gravity. If two identical objects (with the same mass) are placed on different planets, their weights will be different; however, their masses will be still the same. |
- More examples on Newton's third law of motion
| Why both two cows fall down from the cliff? Two cows are standing on a frictionless cliff as shown in the below comics. In order to push the other down from the cliff, one cow pushes a force on the other. Surprisingly, the result is that they both fall down from the cliff. Why? |  | | (Courtesy: Mechanics - Team 28388) | | | | Explanations: If one cow pushes a force on the other, there will a same magnitude force pushing back the cow who pushes the other, as expected by the Newton's third law. Because the two cows are both standing on a frictionless ground, no frictional force resists their motion and they move in the direction same as the force pushing on them, i.e. towards the edge of the cliff. As a result, both the two cows fall down from the cliff. | | Daily examples in Newton's third law of motion We can see many examples of Newton's third law in our daily life. When you hold a water hose with water jet coming out, you would feel a backward force pushing your hand. The third law of motion is also the principle of launching a rocket. The gas coming out from the back of a rocket in a very fast speed pushes the rocket to move forward (i.e. gives the rocket a forward thrust). 
The space shuttle propels into the space by exerting an equal and opposite force with the exhaust gasses. [v] | |
Reference - Parker B., The Isaac Newton School of Driving: physics and your car. Baltimore: The Johns Hopkins University Press, 2003.
- Brain M., "How Car Engines Work" at www.howstuffworks.com.
- Nice K., "How Brakes Work" at www.howstuffworks.com.
- Nice K., "How Crash Testing Works" at www.howstuffworks.com.
- Henderson T., The Physics Classroom at www.physicsclassroom.com.
Glossary -
- Acceleration:
- Change of either the direction or the speed of an object's motion.
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- Crashworthiness:
- It is the measure of how well a vehicle protects its occupants during a crash.
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- Friction:
- The force resists the sliding motion between two surfaces.
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- Hydraulic force multiplication:
- To magnify the force by a hydraulic system consisting of some cylinders with fluid inside.
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- Inertia:
- The resistance of an object to change in state of motion.
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- Internal combustion:
- Ignition of fuel in a small confined space which releases a huge amount of energy in the form of expanding gas.
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- Leverage:
- The force multiplication gained by the use of lever.
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- Mass:
- It is a measure of the inertia of a body. Roughly speaking, mass is the amount of material in a body.
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- Newton's first law of motion:
- It states that each body continues in a state of rest or uniform motion in a straight line, unless a net force acts upon it.
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- Newton's second law of motion:
- It tells us that the acceleration of an object depends solely on two variables, namely the net force acting upon the object and the mass of the object itself.
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- Newton's third law of motion:
- Its states that there is an equal (in magnitude) and opposite (in direction) reaction force for every action force.
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- Power:
- The rate of the amount of work being done over each time interval.
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- Pressure:
- The average amount of force a fluid exerts on unit area of a surface.
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- Torque:
- The tendency for a force to rotate or turn things. It is the turning force.
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- Velocity:
- It tells us that both the speed and direction of an object's motion.
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- Weight:
- It measures the force of gravity acting on an object.
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- Work:
- Work is done whenever a force is exerted on an object so that it moves over a non-zero distance in the same direction as the applied force.
Acknowledgement - PhysicsNet
- The Physics Classroom
- Parker B., The Isaac Newton School of Driving: physics and your car. Baltimore: The Johns Hopkins University Press, 2003.
- NEWS.KAK.NET
- Physics at BHS
- Beyond Books
- Southeast Missouri State Univeristy General Chemistry I Laboratory Home Page
- HowStuffWorks
- Car-Accidents.com
- National Highway Traffic Safety Administration (NHTSA)
- The Science of Gears
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