### Unit 9: Angular Momentum

Why do tornadoes spin so rapidly? The answer is that the air masses that produce tornadoes are themselves rotating, and when the radii of the air masses decrease, their rate of rotation increases. An ice skater increases her spin in an exactly analogous way. The skater starts her rotation with outstretched limbs and increases her spin by pulling them in toward her body. The same physics describes the spin of a skater and the wrenching force of a tornado. Clearly, force, energy, and power are associated with rotational motion. These and other aspects of rotational motion are covered in this unit. We will see that all important aspects of rotational motion either have already been defined for linear motion or have exact analogs in linear motion.

**Completing this unit should take you approximately 16 hours.**

Upon successful completion of this unit, the student will be able to:

- solve kinematics problems involving rotational motion;
- solve dynamics problems involving rotational motion;
- define rotational inertia;
- compare and contrast the dynamics of linear and rotational motion;
- apply energy concepts to rotational motion; and
- give several every-day examples of conservation of angular momentum.

### 9.1: Kinematics of Rotational Motion

Watch this lecture, pausing to take notes, before moving on to the reading below.

Read sections 10.1 and 10.2 of "Chapter 10: Rotational Motion and Angular Momentum". Look over the conceptual questions in each section to test your understanding.

Try solving these practice problems.

Scroll down the page to "Ch 9: Rotational Motion," and select the link to open the PDF document. Read the entirety of Chapter 9. From pages 9-4 to 9-10, solve problems 1-4, 10-13, 16, and 21.

### 9.2: Dynamics of Rotational Motion

Read sections 10.3 and 10.4 of "Chapter 10: Rotational Motion and Angular Momentum". Look over the conceptual questions in each section to test your understanding.

Try solving these practice problems.

Work on solving problems 3 from SAC103 and 1, 2, and 3 from SAC123. The solutions are available, but make a serious attempt to solve the problems before looking at the solutions.

### 9.3: Conservation of Angular Momentum

Read section 10.5 of "Chapter 10: Rotational Motion and Angular Momentum". Look over the conceptual questions in the section to test your understanding.

Try solving these practice problems.

Follow this animation through the various steps until you can convincingly explain what a cat does to land on its feet to a non-physics student.

Work on solving problems 1 through 4 from SAC105. The solutions are available, but make a serious attempt to solve the problems before looking at the solutions.

### Unit 9 Assessment

Take this assessment to see how well you understood this unit.

- This assessment
**does not count towards your grade**. It is just for practice! - You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.

- This assessment