(Before reading this article, let it be noted that physics is an extremely complex subject to teach and grasp. Because of the many complexities involved every day events, I will be writing this article with the impression that the reader knows absolutely nothing on the subject. I will assume that this is a mere introduction to the subject and will write the article on the basis of that assumption. If you are well versed in physics, you will notice that I leave out many details and factors, such as friction, angle of impact, heat energy release, sound energy, etc. I assure you that this was done of purpose in order to get the basic concepts of an automobile accident accross.)
Because of the nature of the subject of this article, there is no real way to start talking about the physics of automobile accidents. One could talk first about the momentum involved in accidents, the exchange of energy between system, or even the role of breaks in decreasing the transferred forces between systems. While all of those subjects pertain directly to the main idea of this article, I think that I would like to start out by saying that automobiles are more dangerous than most people think. Every year, around 40,000 people lose their lives in automobile accidents, due in part to the drivers decisions, but also due to the nature of the physics involved in each accident.
In order to show the physics involved in automobile accidents, let us create a hypothetical situation in which a small SUV crashes into a brick wall.There are two main equations used when discussing inpacts between an automobile and another object; these are the formulas for Force and Momentum.
Force = Mass x Accelleration
Momentum = Mass x Velocity
The reason these two equations are important is because they describe the nature of each “system” before and after the collision. In order to give a general explaination of the involved physics, I will keep the example very simple; this means I will be excluding wind resistance, friction, heat energy, sound energy, etc. To put it in simplest terms, we will be dealing with a closed system collision in which force and momentum are conserved.
Now, as the automobile is moving, we recognice there is no force being applied to it or by it. We can deduce this by noticing that the velocity is constant; this means that there is no accelleration, and therefore the force is zero. As for the momentum, we need to find the car’s mass and plug it, along with it’s velocity, into the equation.
In order to find the mass, we devide the weight of the car by the force of gravity, which is commonly held to be 9.8m/s^2.
Now that we have the car’s mass, and its velocity, we are able to use simple math and determine its momentum. This works out to an extremely large number and is measured in N/s, which is kilogram meters per second.
So now we see how to calculate the cars force and its momentum. What the heck does this mean though? Allow me to explain the signifigance of both values. Momentum can be seen as a measurement of how hard it would be to stop an object of x mass and v speed. If either of those values increases, the momentum would also increase, making the projectile have a greater resistance to opposing forces; which simply means that it would be harder to stop. By using the equation for momentum before and after a collision takes place, we would be able to determine the speeds at which colliding objects move, post collision, or even determine how was a person would move if the two objects were to stop.
This same explaination goes for force. By multiplying the mass of an object and said objects acceleration, we are able to determine nearly the same thigs as momentum; with how much velocity an object post-collision would move, how fast would two objects would move if they were to stick together in a perfectly inelastic collision, and much more.
Now, back to the hypothetical situation. As the car moves with a constant speed, the driver falls asleep, which results in a head on collision between the car and a brick wall. The wall was extremely massive, and therefore did not break into peices or move at all. Rather, it stopped the car dead in its tracks, and roccoched it backwards, propelling the driver through the windshield and onto the road.
Since we know that momentum and force is conserved, we are able to mathematically deduce how fast the driver flew out of the windshield, how fast the car bounced back, and what amount of force the driver experienced. To find the speed at which she was expelled, we would apply the law of conservation of momentum. This law states that in a closed system, momentum will always be conserved. This would mean that the initial momentum would have to equal the final momentum.
Since we see that both the brick wall did not move at all, we assume that the final momentum of the car is equal to its initial. If we were to assume that the mass of the car stayed exactly the same after the crash, then we would simply figure out that the velocity is bounces back with is equal to its origional velocity. If the car were to lose some of its mass during the collision then we would have to increase the value of its velocity in order to follow the law of conservation of momentum.
Because the driver is not attatched to the car, but merely on it, we would be albe to find his momentum by multiplying his velocity by his mass, not including the car. Once the collosion takes place, his body’s natural order would want to continue along the same path at the same speed he was before. Because the car has stopped though, and is actually propelled backwards, the car driver would fly forward with the same velocity.
By applying simple kinematics, we would be able to decipher exactly how far the driver would fly before he hits the ground. That is for another article though.
If we were to use the force equation on the driver, we would be able to see that his body is subject to a tremendous amount of force. Although the driver is not accelerating, his body is keeping a constant speed going. Once the car hits the wall, it will immediatly bounce back with a certain velocity. This change in velocity over the small amount of time can be seen as acceleration!
By taking the acceleration and plugging it into the equation for force, we would be able to discover that the force of the wall on the car is enourmous. In order to conserve force, we would have to assume that the driver is subject to the same force in the opposite direction. By doung this we would assure that the sum of the forces would equal zero.
If we were to venture into a more complex area of physics, we would be able to apply the equations and laws of conservation of energy to our hypothetical situation. Because energy is always conserved, we would be able to determine even more about the reaction between our two objects.
As stated above, this was not meant to analyze, in depth, the physics involved in an accident, but rather to give a glimps of it. Accidents are very complex and very devistating. While I realize that I did not cover all the ins and outs of the physics of accident, I belive that this does an accurate job at “grazing the top” of the subject, and providing readers with an understandable model of the physics involved in accidents.
