Car Accidents And Physics

Every day, many Americans are hurt or killed in car accidents. Many factors can play into an accident. Road condition, mechanical failure, driver error, or simply an 'act of God'? Despite the countless reasons for a car accident, one factor is always present, no matter what the case: physics. Every accident that has ever occurred has involved physics. Using references found in the class text, in science journals, and on the Internet, I will prove this to you.

Take, for example, two cars traveling in opposite directions at 100km/h. One of the drivers dozes off and crosses over the centerline. The two cars hit head on. The driver of car A has remained inside the car and has broken ribs. The driver of car B, however, is on the hood of car A and is pronounced dead at the scene. Cause of death? Disobeying the laws of physics. "Suppose the cars collide head-on and bounce off each other at 4.0 meters per second (9 miles per hour). The collision will change both cars' momentum. But, because no force from outside the system has acted on the cars, their total momentum remains zero"(DiscoverySchool.com). Back to my example, although both cars were going the same speed, one driver lived while the other died. While this may seem like driver A was wearing his lucky tie, probing deeper into the case proves that physics saved his life.

Sir Isaac Newton was the first man to explain what happens in a collision. He proposed the idea that "an object will continue in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it" (Hewitt, 27). This theory is better known as Newton's First Law of Motion, or the Law of Inertia. The driver of car B was not wearing a seatbelt, and as a result, was not connected to the body of the car. According to Newton's first law, an object (the car and its occupants) was moving until its course was changed by an external force (car A). When the collision occurred the car and its occupants changed its motion and direction. However because driver B wasn't attached to the car by his seatbelt he continued with the same speed and in the same direction as the car before the collision. Driver B flew through the windshield and onto the hood of car A. The windshield and car A acted as the necessary external force needed to finally bring driver B to rest. Since the driver of car A was wearing his seatbelt he experienced the same state of motion and deceleration as the car and avoided major injury. However, the quick deceleration did cause his body to be pressed against the seatbelt, thus breaking a few ribs. Again, inertia at work.

Many parents allow their children to ride in the front seat of the car without a seatbelt on. They say that if a collision occurs that they'll just reach over and hold the child back. Many parents don't realize that, "motor vehicle related injury is the leading cause of death for children and young aged 1-24 in the United States"(Smart Motorist-Impaired Driving). If you really look at the physics behind their theory you will see that these parents are dead wrong. Think about the force required to stop a 27kg child traveling at 25m/s. When we combine Newton's second law (F = m * a) with the equation for impulse, we see that momentum is equal to mass times velocity and impulse is equal to the change of momentum (Hewitt 88). Using the derived formula from the previous statement,

F * t = m * Vf - Vi

F*0.4 = 27 * 0 - 25

F = -1687.5N

we see that the force exerted on the child by the seatbelt is 1687.5 Newtons. This is the equivalent of 172.4 kg. This force is only exerted on the body for the time it takes to bring the car to rest, a number we can assume to be approximately 0.4 seconds.

Using Newton's second law, the law that states that acceleration is directly proportional to the force and indirectly proportional to the mass, we can see how the force on an adult traveling at the same speed will experience a different force than that of the child. Using the formula derived from Newton's second law (F = m * a) we see that a 67.5 kg person in the same accident will experience a force of 949.95 kg for 0.4 seconds by their seatbelt. This is a lot of force for one person. So I wonder how those parents with the unbuckled children in the front seat expect to hold back a child exerting 172.4 kg of force while they are trying to hold themselves from crashing into the windshield with 949.5 kg of force.

Newton's Third Law of Motion states that "for every action there is an equal and opposite reaction force" (Hewitt, 71). This law helps explain why a collision at low speed is less serious than one at high speeds. If a car exerts a small force on a brick wall due to low speed then the car and its passengers will experience a force of equal magnitude, but in the opposite direction. If cars of unequal mass collide the more massive car will force the smaller vehicle backwards and the smaller car will experience more force. This is due to the conservation of momentum. The Law of Conservation of Momentum states that in an isolated system the momentum before a collision is equal to the momentum after the collision, if we disregard friction (Hewitt 92, 93). So in the example above we can see that the smaller car gained the momentum lost by the larger car. If two cars of equal mass collide and all the potential energy is converted back into kinetic energy after the collision then the collision is said to be elastic. However if the two bodies collide and stick together and continue with the same final speed then we call it an inelastic collision. Most collisions are neither completely elastic nor inelastic, rather somewhere in between. The conservation of momentum is often very obvious in a collision on a slippery road where friction is lessened, and the effects of the automobiles after the collision is magnified.

For many decades people have used the laws of physics to rebuild the dynamics of an auto accident. Accident reconstruction specialists are people who devoted their careers to using their knowledge of physics and motion along with remains from accident scenes to determine the cause of the accident and how it could have been avoided. They also work with manufacturers to come up with ways to reduce injuries that occur from car accidents. Reconstruction specialists use formulas and basic physics knowledge to determine masses of the vehicle, impact location, rest position, post impact direction which can be found by studying tire marks and gouges in the road. The deceleration from impact to rest is found by looking at post

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