An ice skater performs a fast spin by pulling outstretched arms down and close to her body. What happens to her kinetic energy with respect to the axis of rotation?

Question

BlackTom AI · Accepted Answer

In this scenario, the ice skater converts potential muscle work into rotational kinetic energy as she changes her moment of inertia. 
Option 1: 'It doesn’t change' would be false because angular momentum is conserved in the absence of external torques, but rotational kinetic energy changes when I changes due to the work done by the muscles. 
Option 2: 'It increases' is correct because when she pulls her arms in, I (the moment of inertia) decreases. Since angular momentum L = Iω is conserved (no external torque), ω increases. The kinetic energy for rotation is KE = (1/2) I ω^2; with a smaller I and a larger ω, KE rises overall. 
Option 3: 'It decreases' is incorrect for the same reason: reducing I and increasing ω leads to higher KE, not lower. 
Option 4: 'It depends how fast she rotates' is a misleading statement. While the instantaneous rate of rotation changes as she pulls in, the qualitative outcome is that KE increases due to the required work to shorten the distribution of mass and the resulting change in ω, provided there’s no external torque. 
Thus, the most accurate choice is that the kinetic energy increases when she pulls her arms in during a fast spin.

An ice skater performs a fast spin by pulling outstretched arms down and close to her body. What happens to her kinetic energy with respect to the axis of rotation? 单项选择题

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