Energy and momentum in a car accident

[Jakob Weg]

Two identical cars collide with equal and opposite velocity to each other. What happens to the energy? What happens to the momentum?

Answer: energy is converted into heat and momentum disappears. But what has that to do with force?

 

[haruspex]

Clearly forces are involved, but it can be hard to know exactly what they are at any given time in the collision process, and you don't need to care about that to answer the question.
If you consider the force F_A(t) that car A exerts on car B at time t, and the F_B(t) that car B exerts on car A at time t, you can relate them. You can deduce something about momentum from that.

 

[PWiz]

There are two types of collisions, remember? Also, there is conservation of some quantity here...

 

[CWatters]

Answer: energy is converted into heat and momentum disappears.

I know what you mean but an fussy examiner might be more critical. Momentum is always conserved so how can it disappear?

 

[PWiz]

I know what you mean but an fussy examiner might be more critical. Momentum is always conserved so how can it disappear?

If you carefully consider the initial and final conditions, you'll understand.