Unit 6: Energy and Momentum of Rotating Systems
Physics C: Mech · Unit 6 · Paper 1

Energy and Momentum of Rotating Systems unit test

A test on this unit alone, marked as a percentage and a letter grade — for the test your class is actually sitting, rather than for May. Answer everything, then submit once: seeing the answer to question 3 before attempting question 4 makes the final percentage meaningless.

Paper 1Paper 2Paper 3
Each paper is built from this unit’s 34 terms and is the same for everyone, so a teacher can assign “Unit 6, Paper 1” and every student sits the identical test. Multiple choice is marked objectively; the written sections you mark yourself against the model answer and rubric.
Suggested time 40 min 36 points0/17 attempted
1

Conservation across a collision with a rod

2

Gyroscopic precession

3

Conservation of angular momentum

4

Precession qualitatively

5

Rolling race result

6

Rotational work and power

7

Rotational kinetic energy as the analogue of ½mv²

8

Energy change when I changes

9

Why rotational kinetic energy is not conserved when I changes

10

Rotational work

11

Angular impulse

12

Rotational analogue table

Short answer 1. Define or explain: Angular momentum of a point particle

3 pts

Short answer 2. Define or explain: Why kinetic energy is not conserved when I changes

3 pts

Short answer 3. Define or explain: Rotational power

3 pts

Short answer 4. Define or explain: Ratio of rotational to translational energy

3 pts

Free response

12 pts

A student stands at the center of a horizontal turntable that is free to rotate about a frictionless vertical axle. The student holds a dumbbell in each outstretched hand and is rotating with angular speed ω₀. The student then pulls both dumbbells inward toward the body, reducing the rotational inertia of the student-dumbbell-turntable system from I₀ to I₀/3.

A. Describe the appearance of a graph of the system’s angular momentum as a function of time, from before the student pulls the dumbbells in until after, and justify its shape.

B. Describe the appearance of a graph of the system’s angular speed over the same interval, and derive an expression for the final angular speed in terms of ω₀.

C. Describe the appearance of a graph of the system’s rotational kinetic energy over the same interval, and derive an expression for the final kinetic energy in terms of the initial kinetic energy.

D. Identify the source of any change in kinetic energy, and explain why that does not violate any conservation law.