A physics teacher stands on a freely rotation platform. He holds a dumbbell in each hand of his outstretched arms while a student gives him a push until his angular velocity reaches 1.5 rad/sec. When the freely spinning professor pulls his hands in close to his body, his angular velocity increases to 5.0 rad/sec. What is the ratio of his final kinetic energy to his initial energy?(adsbygoogle = window.adsbygoogle || []).push({});

Basically what I have done is formed the professor into a model of 1) a rod with two spinning dumbells and 2) a cylinder. Using that I was able to use the equations for rotational KE:

[tex] KE_0 = \frac{1}{2} I \omega_{0}^{2} = \frac{1}{2} (mr^2) 1.5^2 = 1.125mr^2 [/tex] and

[tex] KE_f = \frac{1}{2} I \omega_{f}^{2} = \frac{1}{2} (\frac{1}{2} mr^2) 5^2 = \frac{25}{4} mr^2 [/tex].

Dividing the second equation by the first I got a ratio of 25:4.5

However, it never occurred to me that the weight of the teacher and the weight of the dumbbells might have an effect on the outcome of this problem. So basically, how do I do this problem?

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# Homework Help: Rotating platform/MOI

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