# General question about ang momentum throug word problem

1. Dec 1, 2011

### Miike012

a person with mass m stands at the outer edge of a round platform whose radius is r and whose mass is m2. the platform is rotating at a speed of ...rad/s. Assuming the platform to be a free rotating disk, what would be its angular speed if the person walked to its center?

Then the answer book says....

L(i) = L(f) because sum(torque) = 0...

How would I know the sum of the torques is zero...? Is the paragraph said.. " Assuming the platform to be a free rotating disk..." ?

2. Dec 1, 2011

### Staff: Mentor

The torque being referred to is external torques (applied from outside the system under consideration, which consists of the platform and person). If there are no external torques, then the angular momentum of the system must be conserved (i.e. constant).

3. Dec 1, 2011

### Miike012

How do I know there are no external torques? Im thinking there must be some type of torque if the body is rotating....

4. Dec 1, 2011

### Staff: Mentor

Well for one thing the problem statement did not mention any external torques
Presumably the platform got up to speed somehow, but we've "walked in" on the problem some time after things were arranged for us. We're presented with an already rotating platform.

Rotation does not require torque any more than velocity requires force. Remember Newton's first law.