# PHYS 2211 Module 11.3

## Conservation of Angular Momentum

11.3 Conservation of Angular Momentum

### Learning Objectives

By the end of this section, you will be able to:

• Apply conservation of angular momentum to determine the angular velocity of a rotating system in which the moment of inertia is changing
• Explain how the rotational kinetic energy changes when a system undergoes changes in both moment of inertia and angular velocity

### Law of conservation of angular momentum

The angular momentum of a system of particles around a point in a fixed inertial reference frame is conserved if there is no net external torque around that point:

Practice!

Discuss!

You have been asked to help evaluate a proposal to build a device to determine the speed of hockey pucks shot along the ice. The device consists of a rod which rests on the ice and is fastened to the ice at one end so that it is free to rotate horizontally. The free end of the rod has a small, light basket which will catch the hockey puck. The puck slides across the ice perpendicular to the rod and is caught in the basket which is initially at rest. The rod then rotates. The designers claim that knowing the length of the rod, the mass of the rod, the mass of the puck, and the rotational speed of the rod and puck, you can determine the speed of the puck as it moved across the ice.