Chapter 8, Problem 21Q

To determine

The reason why the hips and legs of a shortstop rotate in the opposite direction as compared to his upper body when he throws a ball quickly

Answer to Problem 21Q

Solution:

The angular momentum is conserved. As soon as he leaps from the ground he cannot start rotating in the air. In order to rotate, a balanced equal amount of angular momentum in one direction (hip)and other part (leg)in opposite direction is required to quickly throw a ball into the air.

Explanation of Solution

The reason for this is the principle of conservation of angular momentum. The shortstop’s body has a certain angular momentum to begin with. When the upper part of his body rotates in a particular direction while throwing the ball, the angular momentum of his body tends to increase. To balance it, the lower part of the body tends to rotate in the opposite direction, thus cancelling out the change in the angular velocity of the upper body.