# Physics 312 Lecture: "Conservation of Angular Momentum" Mar 20, 1998

• If a rigid body rotates around a fixed axis, its angular momentum is oriented along the rotation axis
• It often helps to arrange the coordinate axes so that a body rotates in the xy-plane, so that angular momentum is entirely along the z-axis.
• When a rigid body rotates around a fixed axis, its angular momentum around that axis can be expressed as
```             angular momentum = (moment of inertia) * (angular velocity)
```
• When a rigid body rotates around a fixed axis, the derivative with respect to time of its angular momentum around that axis can be expressed as
```             dL/dt            = (moment of inertia) * (angular accleration)
= sum of external torques
```
• If the sum of external torques on a body around some axis is zero, then its angular momentum around that axis is constant.
• If such a body changes its moment of inertia, its angular velocity must change to keep angular momentum constant.

This lecture discusses material in Chapter 11 of Serway.