# Conservation of momentum theory question

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?...

Doc Al
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.

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.

Doc Al
Mentor
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.
That would be a good assumption.

That would be a good assumption.

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.

Doc Al
Mentor
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.
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.

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