A physics student is sitting on a rotating platform. He is holding a heavy weigh

Question

A physics student is sitting on a rotating platform. He is holding a heavy weight in each of his outstretched hands. At request of his physics instructor(!) he carries out various manoeuvres to try to change his angular velocity. Which of the following scenarios are described correctly? Select True or False.

1. A friend throws a third weight so that the student catches it in one of his outstretched hands. No matter what the direction of the throw, the student's angular velocity does not change.

2. Starting with angular velocity gw, he drops the weights. His angular velocity does not change.

3. Starting with angular velocity gw, he raises his feet, and points them outwards from the chair as far as he can. His angular velocity does not change.

4. A friend throws a third weight so that the student catches it in one of his outstretched hands. No matter what the direction of the throw, the student's angular velocity decreases.

5. A friend throws a third weight so that the student catches it in one of his outstretched hands. No matter what the direction of the throw, the student's angular velocity increases.

6. Starting at rest, he drops the weights. His angular velocity increases.

7. Starting with angular velocity gw, he brings the weights in to his chest. His angular velocity does not change.

8. Starting with angular velocity gw, he raises his feet, and points them outwards from the chair as far as he can. His angular velocity decreases.

9. Starting at rest, he drops the weights. His angular velocity does not change.

10. Starting at rest, he brings the weights in to his chest. His angular velocity does not change.

Explanation / Answer

As we know from the conservation of angular momentum that in the absence of external torque.Angular momentum of the system will remain conserved-

J = Iw = Constant

so if moment of inertia of the system increases then angular velocity will decrease-

1. FALSE, here moment of inertia will increase

2. if he drops then MI decreases and w increases

3. in this MI will increase so w will decrease.

4.TRUE

5.FALSE

6. nothing will happen as w = 0 so J = 0

7. here MI will decrease so w will increase

8. TRUE, MI increases here

9. TRUE AS J = 0

10. TRUE AS J = 0 INITIALLY

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