# Homework Help: Conservation of momentum theory question

1. Dec 18, 2012

### Apollinaria

Hi,

I was wondering how one could know when velocity would be in the opposite direction in a momentum question.

For example, two skaters are skating together (as one) at the same speed.
Skater A pushes skater B and B moves at 13m/s. Find the speed of skater A.

I found it and it's the correct answer. But I assumed skater A would still be moving in the same direction as opposed to the opposite direction (after pushing).

So, logic?...

2. Dec 18, 2012

### Staff: Mentor

Recall that momentum is a vector. So if the direction of motion of skater A were reversed, his momentum would be different. Only one solution will conserve momentum.

3. Dec 18, 2012

### Apollinaria

True. But when is it reversed? How do I know when it is reversed?
Edit: Just based on the fact that momentum is conserved? In that case, how do I know when it's conserved? I assumed it always is.

4. Dec 18, 2012

### Staff: Mentor

That would be a good assumption.

5. Dec 18, 2012

### Apollinaria

How do we end up with people/objects going in different directions upon collision then? I've done those types of questions too. Sometimes I get the correct answer and sometimes I don't. I'd like to know a way of recognizing the problem.

6. Dec 18, 2012

### Staff: Mentor

All you need to do is apply conservation of momentum. It will tell you the direction of motion.

That's all you need for this problem. Other problems may be more complicated and might require additional information.

7. Dec 18, 2012

### Apollinaria

Lol, okay, thanks very much for your help :)