# Homework Help: Impulse and Momentum

1. Mar 23, 2004

### Draygon_Phly

Hi! I'm a sixteen year old grade eleven physics student,and I'm having trouble understanding impulse and momentum.I would like you to clear up a few things for me.

"suppose two identical cars traveling at the same speed are brought to a stop: one by its braks, the other by a concrete abutment. In both cases the changes in momentum is the same. the cars have equal masses and experience an equal change in velocity. The impulse to stop each car must be equal too, because impulse and momentum are equivalent quantities.

This is what it says in my text. I don't really understand it, could somone explane this to me? Thank you.

Help!

2. Mar 23, 2004

### Chen

For starters, the book isn't very precise. Impluse doesn't equal momentum, in magnitude or direction. Impluse is the change in momentum, and momentum is defined as the mass of the object times its velocity, i.e $$m\vec v$$.

So to stop a car traveling at $$v$$, you need to change its momentum from $$m\vec v$$ to 0 (the momentum of a resting body is zero since its velocity is zero):
$$\vec F\Delta t = m\Delta \vec v = m\vec v - 0$$
To change the momentum you need to exert a force $$F$$ for $$\Delta t$$ time. You can either exert a very strong force for a short period of time (like the concrete wall would), or you can exert a weaker force for a longer period of time (like the friction would). But no matter what, the product of the force and time must be the same.

You can probably find a lot of sites online that will explain this even better:
Although if you have any specific questions I (and others I'm sure) will be happy to answer them.

Last edited: Mar 23, 2004
3. Mar 25, 2004

### Draygon_Phly

Thanx your lots of help. I don't think I could figure it out with out you.

4. Mar 28, 2004

### expscv

hey thx too that help me recall something