Angular Momentum and Its Conservation
Angular momentum L = I w (or r x p) is conserved when the net external torque is zero.
What is Angular Momentum and Its Conservation?
Angular momentum L = I w (or r x p) is conserved when the net external torque is zero.
Key formula / rule: L = I w; conserved when net external torque is zero.
Key points
- Apply conservation of angular momentum to changing-inertia systems.
- Analyse the energy change when a rotating body changes shape.
Common exam trap
Assuming rotational kinetic energy is also conserved.
Definitions
- Term
Angular Momentum and Its Conservation
- Meaning
Angular momentum L = I w (or r x p) is conserved when the net external torque is zero.
- Term
Angular Momentum and Its Conservation — explanation
- Meaning
The conservation law explains why a spinning skater speeds up on pulling the arms in: I falls, so w rises to keep L fixed, and the kinetic energy rises because the skater does work.
Learning objectives
Apply conservation of angular momentum to changing-inertia systems.
Analyse the energy change when a rotating body changes shape.
Formulae
- Key point
L = I w; conserved when net external torque is zero.
- Key point
I1 w1 = I2 w2 for an isolated changing-shape system.
- Key point
Rotational KE = I w2/2 is not conserved when I changes.
Prerequisites
PHY-U2-ROT-T2-S1-C1
Common mistakes
Assuming rotational kinetic energy is also conserved.
Using L = I w about an axis for which I was not computed.
Keywords
Angular
Momentum
Conservation
Practice preview
What is the SI unit of angular momentum?…
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For angular momentum to be conserved in a system, which of the following conditions must be met?…
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An ice skater spins with her arms outstretched. When she pulls her arms in, her angular speed increases. This phenomenon is an example of the conservation of:…
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