# I really don't understand what I'm even being asked

1. Oct 17, 2011

### 1MileCrash

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

Two skaters, one mass 65kg and one mass 40kg, stand on an ice rink holding a pole of length 10m and negligible mass. Starting from ends of the pole, the skaters pull themselves along the pole until they meet. How far does the 40kg skater move?

2. Relevant equations

3. The attempt at a solution

I don't even know what aspect of physics this is supposed to be about.

EDIT:

I looked at the solution and it says both skaters end up at the center of mass of the system. With that, the problem is trivial.

But to better my understanding, why do both skaters end up at the COM?

2. Oct 17, 2011

### Villyer

My guess is that this is a conservation of momentum problem.

3. Oct 17, 2011

### 1MileCrash

Both skaters end up at the COM, but why?

4. Oct 17, 2011

### SammyS

Staff Emeritus
What is the net force exerted on the system?

5. Oct 17, 2011

### 1MileCrash

ON the system, 0.

6. Oct 17, 2011

### DaveC426913

So, if no net external force is applied to a system, what happens (or doesn't happen) to its CoM?

7. Oct 17, 2011

### DaveC426913

8. Oct 17, 2011

### 1MileCrash

Then the COM cannot change.

But why is it that when they pull, they end up at the COM?

65(10)/105 = 6.2, taking the 40kg "puller" as origin, com is 6.2 meters from it.

9. Oct 17, 2011

### SammyS

Staff Emeritus
If they are very close together, then where is the COM relative to them?

10. Oct 17, 2011

### DaveC426913

Where else would they end up?

Whether 10m apart or 0m apart, if the CoM has not moved, how could they be anywhere else?

11. Oct 17, 2011

### 1MileCrash

Well, I don't know, that's what I'm asking. It only makes sense in my head if we assume that both pullers pull with the exact same force, which seems like an absurd assumption.

12. Oct 17, 2011

### SammyS

Staff Emeritus
But eventually they come together, correct?

13. Oct 17, 2011

### DaveC426913

What if they used completely different forces? What if the 65kg puller did all the pulling and the 40kg puller just hung on? Would that change anything?

Remember, they're on ice. No friction. Newton's First Law applies here.

14. Oct 17, 2011

### 1MileCrash

Wouldn't the center of mass move down the pole in that case?

Or... is it correct to say that the center of mass would move "down the pole" but the center of mass oriented in space, in the room, of the skater/pole system would stay the same? IE if I marked the center of mass on the ice underneath the pole. As the pole moves, the center of mass would still stay above that mark on the ice?

15. Oct 17, 2011

### DaveC426913

Yes. The pole is massless. It is not a useful/meaningful reference point. If one person tugged on it and the other didn't the pole would move relative to the CoM. It's the CoM that acts as the meaningful reference point. And yes, it would remain over the pole.