General question about ang momentum throug word problem

[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..." ?

 

[gneill]

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..." ?

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).

 

[Miike012]

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

 

[gneill]

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

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.

 

[asteriatic]

The assumption that the platform is a free rotating disk means that there are no external torques acting on the system. This is a common assumption in physics problems involving rotation. In this case, the person walking to the center of the platform would not create any additional torque on the system, so the sum of the torques would remain zero. This is why the initial angular momentum (L(i)) would be equal to the final angular momentum (L(f)).