Momentum and impulse: WACE Physical Education Studies Unit 3

Syllabus dot point

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

6 min readUpdated 2 Jun 2026

Quick answer

Momentum is mass times velocity, and impulse is force times the time it acts for. Impulse equals the change in momentum, so to give a ball more speed you apply force over a longer time (follow through), and to land safely you increase the time taken to stop (bend the knees), which lowers the force on the body for the same change in momentum.

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.

Momentum and impulse

Momentum (kg m/s) = mass (kg) x velocity (m/s)

Impulse (N s) = force (N) x time (s)

Impulse = change in momentum, so Force x time = mass x change in velocity.

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.

Same change in momentum, two ways to handle the force

The impulse momentum relationship has a fixed product, force times time, for a given change in momentum. To speed a ball up you want a big change in momentum, so you increase the time force acts (follow through) to build a large impulse. To protect the body you have a fixed change in momentum (you must stop), so you increase the time over which it happens to reduce the force. Same equation, opposite goals.

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.

Using impulse to explain a soft landing

Why landing with bent knees reduces injury risk

A marker wants the impulse momentum relationship applied to safety. Model answer: On landing, the athlete must reduce their downward momentum to zero, which is a fixed change in momentum. Impulse equals force times time and equals that change in momentum. By bending the knees, hips and ankles, the athlete increases the time taken to stop. Because the change in momentum is fixed, increasing the time means the force acting on the body must decrease. A lower peak force across the joints reduces the risk of injury, which is why a flexed, give landing is safer than a stiff legged one.

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).

Confusing which quantity is held constant

The trap is treating force, time and momentum as all free to vary. In generating speed you want maximum change in momentum, so you increase contact time to build impulse. In absorbing force the change in momentum is fixed (you must stop), so increasing time reduces force. Identify what is fixed in the scenario before reasoning, or the explanation will contradict itself.

Exam-style questions

Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

WACE 2022-style6 marks

Define 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 2024-style4 marks

A 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).

Practise this

Sources & how we know this

Physical Education Studies ATAR course: syllabus and support materials — SCSA
Past ATAR course examinations — SCSA