General question about ang momentum throug word problem

AI Thread Summary
In the discussion, a physics problem involves a person walking from the edge to the center of a rotating platform, questioning how to determine the angular speed change. The principle of conservation of angular momentum is highlighted, stating that the initial angular momentum equals the final angular momentum due to the absence of external torques. Participants clarify that the problem assumes no external torques are acting on the system, allowing for this conservation. It is emphasized that rotation can exist without continuous torque, similar to how velocity can exist without ongoing force. The conversation reinforces fundamental physics concepts regarding motion and torque.
Miike012
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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..." ?
 
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Miike012 said:
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).
 
How do I know there are no external torques? I am thinking there must be some type of torque if the body is rotating...
 
Miike012 said:
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 :smile:
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.
 

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