# PHYS 2211 Module 11.2

## Angular Momentum

11.2 Angular Momentum

### Learning Objectives

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

• Describe the vector nature of angular momentum
• Find the total angular momentum and torque about a designated origin of a system of particles
• Calculate the angular momentum of a rigid body rotating about a fixed axis
• Calculate the torque on a rigid body rotating about a fixed axis
• Use conservation of angular momentum in the analysis of objects that change their rotation rate

Why does Earth keep on spinning? What started it spinning to begin with? Why doesn’t Earth’s gravitational attraction not bring the Moon crashing in toward Earth? And how does an ice skater manage to spin faster and faster simply by pulling her arms in? Why does she not have to exert a torque to spin faster?

Practice!

### Angular Momentum of a Rigid Body

Practice!

Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

A ball is attached to one end of a piece of string. You hold the other end of the string and whirl the ball in a circle around your hand.

(a) If the ball moves at a constant speed, is its linear momentum  constant? Why or why not?

(b) Is its angular momentum  constant? Why or why not?