# Why does impulse exist?

1. Mar 18, 2015

### sallychan

Hi guys. I just got a general question.

1. The problem statement, all variables and given/known data
I learned about impulse today in class, and I just wonder why it exists?

Impulse is defined as "the change in momentum", but momentum is always conserved in elastic collision.

2. Relevant equations
So if momentum is conserved, why there is a change in momentum?

3. The attempt at a solution

I am so confused. I will appreciate your help!

Last edited: Mar 18, 2015
2. Mar 18, 2015

### axmls

The momentum of a system is conserved, but that doesn't mean the momentum of a particular object in the system is always the same.

3. Mar 18, 2015

### sallychan

Does it mean that impulse always exist during the collision?

4. Mar 18, 2015

### axmls

Does the object's momentum change? Then there was a net force on it, and that net force occurred for some finite period of time. And so, do you think there was an impulse?

But the distinction is important to make. Momentum is conserved for a system. Take an object that's moving and an object that's sitting still. When they collide, the moving object has a force on it (the normal force from the other object), and it slows down. So that's less momentum in the system. But the other object begins to speed up, because there's a net force on it caused by the normal force from the original moving object, so that's more momentum in the system. All in all, the total momentum of the system remains the same, even though the individual momentums changed.