# Car crash physics question?

So basically today in physics class my teacher asked the whole class a question about car crashes, now I simply need to be corrected or have my hypothesis essentially confirmed.

The question

A car is driving 100 km/h when it slams into a brick wall (this wall has no give)

The same car is driving at 100 km/h when it hits an oncoming car at 100 km/h

which of the two is the worst case scenario.

My thoughts were the head on collision is worse because you add the speeds of the two vehicles together so it would be like hitting the same wall at 200 km/h. Now this is probably a very simple question and you probably would like to know why I'm asking it, Well, most of the class voted for the head on collision, and yet it seemed as though it was a trick question he told the class to think about it when we got home as if perhaps we had all overlooked something.

Is this a trick question or was he trying to confuse us by telling us to give it more thought? Was my explanation correct? If not explain

Also I would assume the cars do not have crumple zones or anything that might minimize the impact.

Relevance

Since the wall has no give, none of the energy of the collision is dissipated by the wall. It is all dissipated by the car.

So in this case it is 1 100mph car's energy spread over one car.

In the second case, it is 2 100mph car's energy spread over two cars. The energy dissipated by each car is the same in all cases.

One other way of thinking about it. Imagine that the two cars come together with impossible symmetry right where a thin sheet of paper is located. Since each car is pushing on the paper, it doesn't move or tear even though it isn't strong enough to resist. Now imagine that you can only see one side of the paper. Can you tell if there is another identical car pushing back or if there is a brick wall pushing back? If not, then the car's response to the collision must be the same.

• Anonymous
8 years ago

It is indeed a trick question by your teacher. Logically you would be correct, but you are not. The forward motion of both cars is immediately stopped by the collision thus ending the force in that direction. Think of it like you pushing against someone as they push back with the exact same force. No one moves either direction correct? 10 lbs one way + 10 lbs the other way = 0 lbs of force in any direction. They equal out thus creating a zero-force point of impact. At the exact moment of collision, if the cars are at the exact same speed and they are of the exact same mass, it would equivalent of hitting a brick wall. The car is being stopped in the opposite direction at the same speed is moving forward. Which is exactly what happens in the brick wall equation. Both collisions have the exact same damage.

• motor vehicle crashes are many times used as coaching approaches to bare the relationship between momentum, potential, velocity and mass. frequently each and all of the kinetic potential (½mv²) is grew to become into warmth except there is a few residual velocity after the crash. Momentum (m*v) is often conserved and equivalent to the final tension*time of a collision.