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How do Newton's three laws of motion explain the way athletes start, change and react to movement?

Apply Newton's first, second and third laws of motion to analyse and explain sporting movements

A focused answer to the WACE Year 12 Physical Education Studies Unit 3 content on Newton's laws of motion. The law of inertia, the law of acceleration linking force, mass and acceleration, and the law of action and reaction, each applied in detail to named sporting examples such as sprinting from blocks.

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

This dot point is examined through application. You should state each law precisely, then explain a sporting situation in terms of forces, masses and accelerations. The marks are in the worked example, not the textbook definition.

Newton's first law: the law of inertia

A body remains at rest, or continues at constant velocity in a straight line, unless acted on by an unbalanced external force. Inertia is the resistance to a change in motion and it increases with mass. A stationary netball will not move until a player applies force to pass it. A heavier athlete is harder to start moving and harder to stop, which is why a large defender takes more force to shift. After crossing the line a sprinter keeps moving and must apply braking force to overcome the inertia of their own body.

Newton's second law: the law of acceleration

The acceleration of a body is directly proportional to the net force applied and inversely proportional to its mass, expressed as force equals mass times acceleration.

A footballer who kicks the ball harder applies more force, so the ball accelerates more and leaves the foot faster. A lighter ball, for the same kick, accelerates more than a heavier one. This law explains why power athletes train both to increase force output and, where rules allow, to optimise the mass being accelerated.

Newton's third law: action and reaction

For every action force there is an equal and opposite reaction force. The two forces act on different bodies. A sprinter in the blocks pushes back and down into the blocks (action), and the blocks push the sprinter forward and up with an equal force (reaction), driving them out. A swimmer pushes water backward and the water pushes the swimmer forward. A basketballer pushes down into the floor to jump and the floor pushes back up.

Combining the laws in one skill

Take a standing vertical jump. By inertia the body stays still until force is applied (first law). The athlete pushes down hard into the floor; by the third law the floor pushes back up. By the second law, the larger this net upward force and the lower the body mass, the greater the upward acceleration and the higher the jump. Linking all three laws to a single movement is the kind of integrated answer top responses give.

Applying the second law with a calculation

Step 1: state the law and formula

Newton's second law: the acceleration of a body is proportional to the net force and inversely proportional to its mass, $F = ma$ , so $a = \frac{F}{m}$ .

Step 2: identify the values from the scenario

A 70 kg high jumper generates a net upward force of 1400 N at take-off. So $F = 1400$ N and $m = 70$ kg.

Step 3: calculate the acceleration

Substitute: $a = \frac{1400}{70} = 20$ m/s squared. This is the upward acceleration produced at take-off.

Step 4: interpret for performance

A larger take-off force, or a lower body mass for the same force, would raise this acceleration and therefore the take-off velocity and jump height. This shows why training targets both increased force production and an optimal power-to-mass ratio, not the slogan of the law alone.

How this maps to the exam

Expect a stimulus image with a command to apply a named law. Identify the body and the forces acting, state the relevant law, then explain the outcome for that athlete using inertia, force and mass, or action and reaction. Specific, force based reasoning is the scoring standard.

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 20216 marksUsing a sprinter driving out of the starting blocks, apply each of Newton's three laws of motion to explain how the athlete begins to move and accelerates forward.

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A 6 mark application needs all three laws translated into the specific forces, masses and accelerations of the start.

First law (inertia): At rest in the set position the sprinter stays still until an unbalanced external force acts; the body's inertia resists the change in motion until the athlete drives against the blocks.
Third law (action and reaction): The sprinter pushes back and down into the blocks (action); the blocks push the sprinter forward and up with an equal and opposite force (reaction). Only the reaction force acting on the athlete moves the athlete.
Second law (acceleration): The acceleration out of the blocks is proportional to this net forward force and inversely proportional to the sprinter's mass ( $a = F/m$ ); a larger drive force and a lower body mass give greater forward acceleration.

Markers reward inertia before movement, the correct action/reaction pair on the blocks, and the force-mass-acceleration link, all tied to the sprinter.

WACE 20234 marksAn athlete of mass 80 kg generates a net horizontal force of 640 N when driving off the ground. Calculate the resulting horizontal acceleration, then explain how Newton's second law predicts the effect of the athlete reducing body mass while maintaining the same force.

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A 4 mark answer needs the calculation and an interpretation using the second law.

Calculation: Rearranging $F = ma$ gives $a = \frac{F}{m} = \frac{640}{80} = 8$ m/s squared.
Interpretation: Newton's second law states acceleration is inversely proportional to mass for a fixed force. If the athlete reduced mass while applying the same 640 N, the acceleration would increase (for example, at 64 kg, $a = \frac{640}{64} = 10$ m/s squared).
Sporting meaning: This is why power athletes aim to maximise force output while optimising (not simply minimising) the mass being accelerated.

Markers reward the correct acceleration of 8 m/s squared, the inverse mass relationship, and a sensible sporting interpretation.