Answer to Question #174053 in Physics for Andu

Question

a) Why does the ballerina's angular velocity increase when she, being in a rotating motion, folds her arms? b) Why does its angular velocity decrease when, afterwards, it spreads its wings? - Instruction: Considering the Earth-ballerina system closed, you can apply the law of conservation of momentum, or as it is said, of kinetic moment, so L1 = L2

Answer 1

The  ballerina stretches out her hands to slow down from spinning. It is based on the principle of conservation of angular momentum.

An object (the  ballerina) rotating about an axis is the product of its momentum of inertia and its angular velocity (the speed of rotation and the orientation of the axis which the rotation takes place). The physical equation is L=Iw.

When the  ballerina is spinning in a closed system, and no external forces are applied to it, it will have no change in angular momentum.

The conservation of angular momentum explains the angular acceleration of a ballet dancer as she brings her arms and legs close to the vertical axis of rotation.