AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Applications may include examples from sport.
Full specification wording
Angular momentum (A-level only): Applications may include examples from sport.
Angular momentum L = Iω is conserved when the net external torque is zero.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Angular momentum L = Iω is conserved when the net external torque is zero. In sport, this explains why a spinning skater or diver can change angular speed by altering body configuration. Pulling limbs in reduces the moment of inertia I, so ω must increase to keep L constant; extending limbs increases I and reduces ω. The rotational kinetic energy E = ½Iω² changes because internal work is done, even though L is conserved. They should also relate the effect to real sporting techniques, such as a gymnast tucking during a somersault or a diver tightening before entry, and explain the role of frictional torque in slowing rotation.
What earns the marks
- States that angular momentum is conserved when the net external torque on the system is zero.
- Uses L = Iω to compare the moment of inertia before and after a change in body shape.
- Explains that reducing the moment of inertia increases angular velocity, and increasing it decreases angular velocity.
- Applies conservation of angular momentum to a named sporting action, such as a skater pulling in their arms or a diver tucking.
- Recognises that rotational kinetic energy changes because internal work is done, even though angular momentum is conserved.
- Discusses how frictional torque from air resistance or contact surfaces reduces angular momentum over time.
Common mistakes and how to put them right
Treating angular momentum as conserved even when a significant external torque acts.
Put it right: Check that the net external torque is zero before applying conservation.
Confusing moment of inertia with mass alone.
Put it right: Use I = Σmr² and recognise that distribution of mass affects I.
Assuming rotational kinetic energy is conserved when a skater pulls their arms in; the correction is to state that internal work increases kinetic energy while angular momentum stays constant.
Exam tips
- Begin by stating the condition for conservation: net external torque is zero.
- Show the before-and-after equation I₁ω₁ = I₂ω₂ and substitute values carefully, keeping units consistent.
- When explaining a sporting example, name the action and describe how the body configuration changes I and ω.
Try the lesson
The start of the Angular momentum (A-level only): Applications may include examples from sport. lesson, made from checked teaching notes for this specification point.
Have a go first
A figure skater spinning on the spot pulls her arms in tightly, so her moment of inertia about the vertical axis falls. No external torque acts about that axis. What happens to her angular velocity?
Listen
Angular momentum L = Iω is conserved when the net external torque is zero. In sport, this explains why a spinning skater or diver can change angular speed by altering body configuration. Pulling limbs in reduces the moment of inertia I, so ω must increase to keep L constant; extending limbs increases I and reduces ω. The rotational kinetic energy E = ½Iω² changes because internal work is done, even though L is conserved. They should also relate the effect to real sporting techniques, such as a gymnast tucking during a somersault or a diver tightening before entry, and explain the role of frictional torque in slowing rotation. A strong answer earns marks for these points. States that angular momentum is conserved when the net external torque on the system is zero. Uses L = Iω to compare the moment of inertia before and after a change in body shape. Explains that reducing the moment of inertia increases angular velocity, and increasing it decreases angular velocity. Applies conservation of angular momentum to a named sporting action, such as a skater pulling in their arms or a diver tucking. A common slip: Treating angular momentum as conserved even when a significant external torque acts. Instead, check that the net external torque is zero before applying conservation. Exam tip: Begin by stating the condition for conservation: net external torque is zero.
Key words
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Angular momentum
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Moment of inertia
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Rotational kinetic energy
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Frictional torque
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See every definition
- Angular momentum
- L = Iω, and it is conserved when the net external torque is zero.
- Moment of inertia
- Reducing it increases angular velocity, and increasing it decreases angular velocity, to keep L constant.
- Rotational kinetic energy
- E = ½Iω², and it changes because internal work is done, even though L is conserved.
- Frictional torque
- Torque from air resistance or contact surfaces that reduces angular momentum over time.
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