A student stands on a frictionless rotating platform while holding two identical…

PHYSICS · Multi-part · CBSE · Physics · System of Particles and Rotational Motion · System of Particles and Rotational Motion

Problem

A student stands on a frictionless rotating platform while holding two identical

2.0\,\text{kg}

masses at arm’s length, each a distance of

0.75\,\text{m}

from the rotation axis. The combined moment of inertia of the student and platform (excluding the hand-held masses) is

1.5\,\text{kg\,m}^2

. Initially the system rotates at

1.2\,\text{rev\,s}^{-1}

. The student then pulls the masses straight inward until they are only

0.25\,\text{m}

from the axis. Assume the pull takes

2.0\,\text{s}

and that any external torque on the system is negligible. (a) Calculate the new rotation frequency of the system once the masses reach the

0.25\,\text{m}

radius. (b) Determine the change in rotational kinetic energy of the entire system over the pulling process. (c) Explain, in words, the physical origin of the energy change you found in part (b). (d) Estimate the average power the student must supply while pulling the masses inward.

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