# 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!!

@doug.walker

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

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

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.