A physics student is sitting on a frictionlessplatform that is rotating with an

Question

A physics student is sitting on a frictionlessplatform that is rotating with an angular speed ofi = 1.9 rad/s. Hisarms are outstretched, and he is holding a heavy weight in eachhand. The moment of inertia of the student, the extended weights,and the platform about the rotation axis is Ii =8.8 kg·m2. When thestudent pulls the weights inward toward his body, the moment ofinertia decreases to If =5.2 kg·m2. a. What is the resulting angular speed,f, of the platform? b. By how much is the kineticenergy of the system increased? (Think about where this increase inenergy comes from!)

Explanation / Answer

By using conservation of angular momentum we have
          i Ii = fIf         =>f = 1.9 * 8.8 / 5.2
                  = 3.215 rad/s The kinetic energy increased
           E= Iff2/2 -Iii2/2
             = 26.88 - 15.884
             = 10.996 J
This increasing is caused by the work done by thestudent pulls the weights inward toward his body. This increasing is caused by the work done by thestudent pulls the weights inward toward his body.

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