Collision equations

hey there,

I’m struggling with understanding which equation to use for different types of collisions.

any advise would be a huge help as my exam is in only a few weeks.

many thanks!!


Hi Sarah,

Did you have a specific question in mind?

In Physics you are expected to know about two types of collisions: elastic and inelastic.

Elastic collisions involve both conservation of momentum and kinetic energy. These only really happen at the atomic, or sub-atomic level, and are unlikely to come up in your exam.

Inelastic collisions involve the conservation of momentum, but kinetic energy will be lost.
p=mv is one equation for momentum. A question using this formula might involve a collision and expect you to:

  • work out the total momentum before the collision

  • Recognise that the total momentum before will be the same as the total momentum after the collision

  • Use the equal momentum value to work out the mass or velocity of an object that is part of the collision

Here is an example with a worked solution

Another equation you could be expected to use is Δp=Ft
This is the change in momentum (also known as impulse) is equal to the Force multiplied by time of collision.

It may be important to note that a change in direction of velocity here is important, so a ball hitting a wall at 20 metres per second, then rebounding at 8 metres per second, would have a change in velocity of -28 metres per second. Think of it as +20 in one direction, then -8 in the other direction, so the change is final - initial = -28

Here’s an example of an impulse question.
And another one.

If you still need some clarification, let me know a couple of example problems and I’ll give you step by step guidance.

Hi Sarah,

Here is a video tutorial for the roller skating question. Let me know if you have any questions after watching it.

Then see if you can have a go at the Ice Skating one. Think about what Roy’s initial momentum will be (remember he is stationary). If you still struggle, I’ll upload a video tutorial for that one as well.