A spinning ice skater draws in her outstretched arms to reduce her moment of inertia by a factor of two. Determine the ratio of final kinetic energy to her initial kinetic energy

Question

nottim · Answer

HEllo?

anonymous · Answer

Assuming no friction with ice and no friction with air, then what changes? Have a look at the energy conservation principle and find yourself what must happen with speed. In fact this is a technique used by skaters to increase their spinning speed.

vincent-lyon.fr · Answer

Her KE will double.
Work out her new rotation velocity first, using appropriate theorem, then apply expression for KE.

anonymous · Answer

@Vincent-Lyon.Fr, I am afraid KE is not double. It remains the same, therefore reduction in Inertia has to be compensated by increase in spinning speed

vincent-lyon.fr · Answer

@CarlosGP You are mistaking angular momentum and kinetic energy.

Conservation of KE does not apply here, because the ice skater in not a rigid body (the increase in KE is provided by work done by her muscles).

anonymous · Answer

@Vincent-Lyon.Fr
And you are totally right. Thanks for the clarification!