# Calculate Mass of An Object

1. Feb 13, 2010

### javas1

1. The problem statement, all variables and given/known data
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

2. Relevant equations

I do not have an equation.

3. The attempt at a solution

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

2. Feb 13, 2010

### Staff: Mentor

Hint: What's conserved in any collision?

3. Feb 13, 2010

### 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?

4. Feb 13, 2010

### sean-820

I think thats 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.

5. Feb 13, 2010

### javas1

Unless the other guy was also 200 lbs, right?

6. Feb 13, 2010

### Staff: Mentor

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.

7. Feb 13, 2010

### javas1

That's perfect. Thanks!