Question

Typed please. An ice skater can change the speed of her rotation by bringing her arms close to her body. Because her angular momentum is approximately conserved, why does she rotate faster? Before she starts spinning, how does she generate rotation in the first place?

Answer 1

As per the conservation of angular momentum,

The product of initial moment of inertia and initial angular velocity is always equal to the product of final moment of inertia and final angular velocity.

Mathematically,

I1 w1 = I2 w2

When an ice skater brings are closer to the body its moment of inertia decreases and hence the angular velocity w2 increases so as to keep the angular momentum constant.

Before she start spinning she generates rotation in the first place by applying the tangential force which results into a net torque, thus giving rotational kinetic energy