Calculating Mass in a Momentum Conservation Problem

[javas1]

Homework Statement

A runner has a mass of 168 Kg and is traveling North at 5 meters per second. Another runner is traveling South at 5 meters per second. They collide and come to a complete stop. What is the mass of the second runner

Homework Equations

I do not have an equation.

The Attempt at a Solution

I have stayed up all night trying searching on the Web to find out anything.

 

[Doc Al]

Hint: What's conserved in any collision?

 

[javas1]

Yes, it is momentum, I believe.

I read that if two objects crash into each other, then they bounce off each other - and that is not a complete stop.
However, it seems that the mass of both objects would need to be equal, as long as the velocity is equal.

Is that right?

 

[sean-820]

I think that's right. If one was 200lbs then that one would blow through the other guy and not stop at the point of collision like i think the question is saying.

 

[javas1]

I think that's right. If one was 200lbs then that one would blow through the other guy and not stop at the point of collision like i think the question is saying.

Unless the other guy was also 200 lbs, right?

 

[Doc Al]

Yes, it is momentum, I believe.

Right.

I read that if two objects crash into each other, then they bounce off each other - and that is not a complete stop.

They may or may not bounce off, but no matter what, momentum will be conserved. Here you are told that they come to a stop, so you know they don't bounce off.

However, it seems that the mass of both objects would need to be equal, as long as the velocity is equal.

Is that right?

Yes, exactly right. Since they come to a complete stop, you know that the total momentum is zero. That can only be true if their initial momenta are equal and opposite. Since they have the same speed, the only way they can have the same momentum is if they have the same mass.

 

[javas1]

Right.


They may or may not bounce off, but no matter what, momentum will be conserved. Here you are told that they come to a stop, so you know they don't bounce off.

Yes, exactly right. Since they come to a complete stop, you know that the total momentum is zero. That can only be true if their initial momenta are equal and opposite. Since they have the same speed, the only way they can have the same momentum is if they have the same mass.

That's perfect. Thanks!