Question

Hi, may I get some help with explanation? Thank you

Problem 3 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. Starting with angular velocity wow he raises his feet, and points them outwards from the chair as far as he can. His angular velocity increases. Starting at rest, he drops the weights. His angular velocity decreases. Starting with angular velocity wgw he raises his feet, and points them outwards from the chair as far as he can. His angular velocity reverses. Starting with angular velocity wow he brings the weights in to his chest. His angular velocity does not change. Starting with angular velocity wow he drops the weights. His angular velocity does not change. Starting with angular velocity wow he transfers both the weights to one outstretched hand. His angular velocity (after the manoeuvre is complete) has reversed. Starting with angular velocity wgw he raises his feet, and points them outwards from the chair as far as he can. His angular velocity decreases. Starting with angular velocity wgw he brings the weights in to his chest. His angular velocity decreases. Starting at rest, he brings the weights in to his chest. His angular velocity increases. Starting at rest, he drops the weights. His angular velocity does not change.

Answer 1

Since there is no external torque angular momentum of the system is conserved which is given by Iw. Now I (moment of inertia ) depends on spread of mass . Greater is the spread of mass away from axis more Is the I value .

1) false . Since raising his leg and stretching outward spreads the mass away from the axis , I increases hence w decreases .

2) false , dropping weight does not change moment of inertia.  

3) false , it can decrease the w not change the direction of w

4) false , spread of mass changes hence I and w changes

5) false , net mass changes , hence w changes .

6) false ,

7) true

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