Choose the system and axis
Decide whether external torque can change the selected system’s total angular momentum.
AP Physics 1 · Unit 6 · Energy and Momentum of Rotating Systems
Conservation of Angular Momentum
Move mass toward a rotation axis, follow a complete orbit, and test angular-momentum conservation in a sticking turntable collision.
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Learn by doing
Three ideas you’ll put into motion
Compare inertia and angular speed
Observe a changing mass distribution and explain the work involved in a skater’s faster spin.
Predict and test an interaction
Combine initial angular momenta, calculate the final rotation, then choose a putty speed that stops the turntable.
The big idea
The key ideas, at a glance.
External torque determines the change
With zero net external angular impulse about a chosen fixed axis, total angular momentum is constant. For a shared rotation, Iiωi = Ifωf.
Conservation questions are separate
A system can conserve angular momentum while its angular speed changes. Kinetic energy need not stay constant, and linear momentum requires its own external-force condition.
A quick comparison
Move the same mass inward while keeping net external torque about the axis negligible.
Smaller rotational inertia means greater angular speed at the same angular momentum.
Teaching this topic?Classroom fit, AP connections, and support
- Classroom fit
- Use for a concept introduction, guided independent practice, or a projected class discussion.
- Before students begin
- Angular momentum, angular impulse, rotational inertia, rotational kinetic energy, torque, and orbital motion.
- Built-in support
- Prepared scenes, guided prompts, and feedback on lesson questions.
- AP Physics 1 connections
- Unit 6, Topic 6.4: conservation of angular momentum and system selection (objectives A and B), with an orbital connection to Topic 6.6.
- Check for understanding
- Ask why a fixed axle can change linear momentum without changing angular momentum about the axle, and why a skater’s kinetic energy can increase at fixed angular momentum.
Ready to test your thinking?
The first scene is set up. Start the lesson and explore.