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How do momentum and impulse explain why follow through increases ball speed and why bending the knees softens a landing?

Define momentum and impulse and apply the impulse momentum relationship to generate force and to absorb force safely in sport

A focused answer to the WACE Year 12 Physical Education Studies Unit 3 content on momentum and impulse. Momentum as mass times velocity, impulse as force times time, the impulse momentum relationship, and how follow through generates speed while bending joints on landing reduces force for safety.

Generated by Claude Opus 4.86 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

WACE wants the two definitions, the relationship between them, and two applications: generating speed and absorbing force. The strongest answers manipulate the relationship to explain why technique changes the force or velocity.

Momentum

Momentum is the quantity of motion a body has, calculated as mass times velocity. Because an athlete's mass is essentially fixed during a skill, momentum is changed by changing velocity. A heavier, faster rugby player carries more momentum and is harder to stop than a lighter, slower one.

Impulse

Impulse is the force applied multiplied by the time over which it acts. The impulse momentum relationship states that the impulse applied to a body equals its change in momentum. This single relationship explains both generating and absorbing force, because it can be read two ways: a force over a time produces a velocity change, and a required velocity change can be spread over a longer time to reduce the force.

Generating force: follow through

To give a ball maximum speed, the athlete maximises the impulse applied to it. Following through keeps applying force to the ball over a longer time before release or after contact, increasing the impulse. A larger impulse means a larger change in momentum, and since the ball's mass is fixed, that means a higher release velocity. This is why a long follow through in a throw, a tennis serve or a golf drive is not decoration but a way to extend the contact time and increase the impulse.

Absorbing force: safe landings and catching

When landing from a jump or catching a fast ball, the body must lose a fixed amount of momentum (stopping the motion). By increasing the time over which this happens, the force experienced is reduced, because force times time is fixed. Bending the knees, hips and ankles on landing lengthens the stopping time and lowers the peak force on the joints. Drawing the hands back while catching a hard ball does the same. This is the basis of safe technique and injury prevention.

How this maps to the exam

Questions either ask how an athlete maximises ball speed or how they reduce force on landing or catching. State the relevant definitions, quote that impulse equals change in momentum, then explain whether time is being increased to build impulse (for speed) or to reduce force (for safety).

Exam-style practice questions

Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WACE 20226 marksDefine momentum and impulse, then use the impulse-momentum relationship to explain both why a golfer follows through on a drive and why a gymnast bends the knees on landing.

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A 6 mark answer needs the two definitions and the relationship applied to generating and absorbing force.

Definitions: Momentum is mass times velocity ( $p = mv$ ). Impulse is the force applied multiplied by the time it acts ( $impulse = F \times t$ ), and impulse equals the change in momentum.
Generating force (golf follow-through): A longer follow-through increases the time the club applies force to the ball (a larger time in $F \times t$ ), so a greater impulse produces a greater change in momentum and a higher ball velocity at impact.
Absorbing force (gymnast landing): Bending the knees increases the time over which the body's momentum is brought to zero. For the same change in momentum, increasing the time reduces the force ( $F = \frac{\text{change in momentum}}{t}$ ), so the landing is softer and injury risk falls.

Markers reward both definitions, longer contact time increasing impulse to build momentum, and longer stopping time reducing force on landing.

WACE 20244 marksA 0.5 kg ball is accelerated from rest to 20 m/s. Calculate the change in momentum, and explain how the athlete could increase the ball's final velocity through the impulse-momentum relationship.

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A 4 mark answer needs the calculation and an interpretation.

Calculation: Change in momentum equals final momentum minus initial momentum: $\Delta p = (0.5 \times 20) - (0.5 \times 0) = 10$ kg m/s.
Increasing velocity: Because impulse equals change in momentum ( $F \times t = \Delta p$ ), the athlete can increase the ball's final velocity by applying a larger force, or by applying the force over a longer time (a longer follow-through), or both.
Link: A greater impulse produces a greater change in momentum, and for a fixed mass that means a higher final velocity.

Markers reward the correct change in momentum of 10 kg m/s and a valid way to raise velocity via more force or more time (impulse).