A figure skater stands on one spot on the ice (assumed frictionless) and spins a

Question

A figure skater stands on one spot on the ice (assumed frictionless) and spins around with her arms extended. When she pulls in her arms, she reduces her rotational inertia and her angular speed increases so that her angular momentum is conserved Compared to her initial kinetic energy, her rotational kinetic energy after she has pulled in her arms must be (pick the best answer and fully explain your reasoning using complete sentences) (1) the same (2) larger because she is rotating faster (3) smaller because her rotational inertia is smaller 5.

Explanation / Answer

Formula for Rotational kinetic energy

KEr = (1/2)lw^2

or KEr = L^2 / 2*l

where, L = angular momentum

l = rotational inertia = mr^2

As she pulles in her arm, so distance r is decreased

rotational inertia is decreased

L is conserved.

so, rotational KE is larger than initial KEr.

Option (2) is correct (Larger because she is rotating faster).

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