How does rotation work in sport, and how do athletes control their spin?
Angular motion in sport, the quantities of angular motion, moment of inertia and its effect on angular velocity, and the conservation of angular momentum.
A focused WJEC A-Level PE answer on angular motion, covering torque, moment of inertia, angular velocity and angular momentum, and how the conservation of angular momentum lets performers control their rate of spin.
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What this dot point is asking
WJEC wants you to describe angular (rotational) motion in sport, define the angular quantities, explain moment of inertia and how mass distribution changes it, and apply the conservation of angular momentum to show how athletes control their rate of spin.
What starts and describes rotation
A diver, gymnast or skater begins to rotate when an off-centre (eccentric) force applies a torque about an axis. Once rotating, the motion is described by three linked quantities: angular velocity, moment of inertia and angular momentum.
Moment of inertia
Because athletes cannot change their mass mid-movement, they control moment of inertia by changing body shape. A tucked somersault has a small moment of inertia (mass close to the axis); a straight or piked somersault has a larger one (mass further from the axis).
Conservation of angular momentum
This is the most important idea in the topic. In flight a gymnast cannot change their angular momentum, but they can change their shape. Tucking reduces moment of inertia, so angular velocity rises and they rotate faster; opening out increases moment of inertia, so angular velocity falls and they slow the rotation to control the landing.
Examples in context
Example 1. The trampolinist's straight somersault. A trampolinist keeping a straight body shape rotates slowly because the extended mass gives a large moment of inertia, which is why straight somersaults score for difficulty: they require more angular momentum at take-off. This shows shape choice trades speed for style.
Example 2. The spinning skater finale. The classic scratch spin starts slowly with arms out, then accelerates dramatically as the skater pulls in. Because the ice provides almost no external torque, angular momentum is conserved and the speed-up comes entirely from the drop in moment of inertia, a textbook WJEC application.
Try this
Q1. Define angular momentum and give the equation for it. [2 marks]
- Cue. The quantity of rotation of a body; angular momentum equals moment of inertia multiplied by angular velocity.
Q2. Explain why a tucked somersault rotates faster than a straight somersault. [3 marks]
- Cue. Tucking brings the mass close to the axis, lowering the moment of inertia; with angular momentum conserved, the angular velocity must rise, so the rotation is faster.
Q3. State the condition under which a performer's angular momentum is conserved. [1 mark]
- Cue. When no external torque acts on them (for example, once airborne, or on near-frictionless ice).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20196 marksExplain, using the conservation of angular momentum, how an ice skater increases their rate of spin during a routine.Show worked answer →
Angular momentum is the quantity of rotation of a body and equals moment of inertia multiplied by angular velocity. Once the skater is rotating, angular momentum is conserved because there is no significant external torque (the ice is almost frictionless).
Moment of inertia depends on the mass of the body and how that mass is distributed around the axis of rotation: the further the mass is from the axis, the larger the moment of inertia.
When the skater pulls their arms and legs in close to the axis, the moment of inertia decreases. Because angular momentum is conserved (constant), the angular velocity must increase, so they spin faster.
To slow down, they extend their arms and legs, increasing the moment of inertia, so the angular velocity falls.
Markers reward the definition of angular momentum, conservation (no external torque), the effect of mass distribution on moment of inertia, and the inverse relationship between moment of inertia and angular velocity.
WJEC 20213 marksDefine moment of inertia and state two factors that affect its size.Show worked answer →
Moment of inertia is the resistance of a body to a change in its state of angular motion (its reluctance to start or stop rotating).
It is affected by the mass of the body: a greater mass gives a greater moment of inertia.
It is also affected by the distribution of that mass relative to the axis of rotation: the further the mass is from the axis, the greater the moment of inertia.
Markers reward a correct definition and both factors (mass, and the distribution of mass about the axis).
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