The Laws of Physics and How They Affect Driving

Increased speed makes the laws of physics become more and more important to the driver. These laws, although not enforced by a policeman or written by a law producing body, are absolutely binding on all drivers, and no one can relax their effect. The laws of physics control any and every object that moves. The particular laws which apply to driving cover areas such as friction, centrifugal force and inertia, impact, and gravity. You should always remember that these laws apply to city driving as well as to driving on the highway, because their significance increases proportionately with the speed at which you are travelling. However, this article on highway driving would seem the best place to discuss their importance.

Proper sight requires sufficient light and time for a picture to impose itself on the retina of the eye, be relayed to the brain, and thereby trigger a reaction by the driver. This means that traffic signs, signals, and pavement markings become increasingly important when driving at higher speeds. These give the driver advance warning of any curves, hills, intersections, or railway crossings that may be ahead, as well as intended maneuvers by other drivers. A driver must learn to recognize all signs and signals instantly, for at higher speeds the time for recognition and reaction becomes shorter and shorter. To facilitate recognition, signs and signals throughout the United States are standardized by shape and color and can reflect light to be seen at night. Remember, it takes time to observe a sign, signal, or condition, and then react to it. Reaction time for a given driver is fairly constant, but the distance travelled in this time is directly related to speed. Therefore, stopping distances and distances required for evasive action become greater as speed is increased.

Friction

Friction is the force which opposes the motion of one surface over another, and is the means through which a vehicle may move in a straight line, or may turn or stop. This force is exerted entirely through four small friction areas, also known as, tires. If we assume that the average reaction time is 0.75 seconds than common sense tells us that the faster the car is travelling, the greater the distance it will take to stop. The difference in stopping distance from 40 miles per hour to 70 miles per hour is approximately 3.5 times greater. This means that if you can stop within 100 feet at 40 miles per hour, you will need 350 feet of leeway to stop travelling at 70 miles per hour.

These conditions only occur part of the time, however: should the force of friction be reduced by ice, snow, rain, oil, mud, loose gravel, a rough surface, or poor tires, then stopping distances will increase drastically and evasive maneuvers will become much more difficult, or even impossible. Because stopping distance increases more rapidly than speed, it is important to allow a greater distance between your car and the car in front as your speed increases.

Inertia and Centrifugal Force

If at any time the frictional force, or traction, between the four small areas of the tire and road surface are lost, control is lost as well, and one or both of the following physical forces may determine the situation: inertia, the tendency of a moving body to keep moving in a straight line unless an outside force acts to change its direction of motion; and centrifugal force, the tendency of a moving body turning about a center to fly away from that center. Centrifugal force can be demonstrated by placing a weight on the end of a string and swinging it in a circular motion. If the string is release or breaks, the weight will leave the circular path and continue in a straight line.

Obviously, a similar effect can happen to a turning vehicle. A car driving around a curve must overcome the centrifugal force in order to make the turn. If the centrifugal force is greater than the friction between the tires and the road, the car will not be able to turn, but will skid off the highway. The key point is that the friction increases with speed, but the centrifugal force increases even more rapidly. Therefore, the faster your speed, or the sharper the turn, the greater is the chance that you will be unable to get around safely. If you remember this principle, you will realize that you must slow down before entering a curve, especially if the road is slightly slippery

Brakes should never be applied after entering a curve, as this has a tendency to reduce the friction between the wheels and the road. Remember, friction enables you to move your car, control it, and stop it. When you consider that for each tire the area touching the road surface is about equal to the size of your hand, it is understandable that many factors can cause loss of friction, and resultant loss of control. The greater the speed, the greater the possibility this may happen – and the greater the consequences. Speed must always be adjusted to suit road conditions.

As well as the speed of the car, another factor determining whether or not you will be able to make a turn safely is the angle at which the road is banked through the curve. The easiest is a banked turn (similar to a race track); the second, a flat road surface; and the third, a crowned surface. The flat road surface is dangerous at high speed, and in comparison, in a turn, the crowned surface can only be negotiated at low speeds because the car is tilted against the direction of the curve. On entering sharp curves, there is usually an advisory speed sign posted, telling you the speed at which the curve may be safely taken. One who ignores these signs is indeed a very foolish driver.

Kinetic Energy and the Force of Impact

If control of a car is lost, the usual result is collision, either with another car or with a fixed object. The all important variable in this situation is the force of impact. The force of impact itself is a function of the speed and the weight of the car. If you double the speed of a car before a collision, the force of impact is four times as great. If you triple the speed of the car before collision, the force of impact is multiplied nine times! Weight also has a part to play here; if the weight of the vehicle doubles, the force of impact doubles too. The total result of doubling the speed and the weight of the vehicle would be to increase the force of impact eight times. Therefore, any collision would necessarily be eight times as damaging. In effect, the impact of hitting a solid object at 30 miles per hour is like driving off a three story building.

Highway engineers use several techniques to reduce the force of impact in cases of unavoidable contact with surrounding objects. Smooth metal guard rails allow a car to glance off rather than hit solidly. Wide road shoulders, free of obstacles such as trees, culverts, and bridge abutments, help to reduce the hazard. Where light and sign standards are essential, these poles are designed to sheer or break off easily on contact. The best way to make sure that the force of impact does not act upon your car is to drive at all times in a manner which will avoid collision with any and all objects!

The Force of Gravity

Gravity – the force which attracts objects downwards towards the center of the earth – will cause cars to lose speed going up hills, thereby decreasing their stopping distances; and to accelerate going down hills, thereby increasing their stopping distances.

A good driver will cut his speed when descending a hill; on steep grades, he should put his gear selector into low, so that the engine of the car will act as a brake. Hills are potential driving hazards for other reasons also. They limit visibility; the driver should not pass on or approaching a hill, no matter how slowly the vehicles in front are moving, unless there is a passing lane. At the crest of a hill, the driver must be alert for approaching cars not in their proper lane, or for obstacles in the road ahead, such as a car stopped while waiting to make a left turn. Remember that you must be able to stop your vehicle in the distance you can see ahead either day or night.