Newton's laws of motion: WACE Physical Education Studies Unit 3
“Apply Newton's first, second and third laws of motion to analyse and explain sporting movements”
Newton's first law (inertia) says a body keeps its state of motion unless an unbalanced force acts on it. The second law (acceleration) says force equals mass times acceleration, so more force or less mass means more acceleration. The third law (action and reaction) says every force has an equal and opposite force, which is how athletes drive off the ground. WACE wants each law applied to a named skill.
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.
Force (N) = mass (kg) x acceleration (m/s squared)
Rearranged, acceleration = force / mass.
So for a fixed mass, a larger applied force produces a larger acceleration; for a fixed force, a smaller mass accelerates faster.
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.
The action and reaction forces are equal in size and opposite in direction, but they never cancel out, because they act on two different objects. The athlete's push acts on the ground, and the ground's reaction acts on the athlete. Only the force on the athlete moves the athlete. Saying the forces cancel is a classic misunderstanding that loses marks.
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.
Markers want the mechanics, not the slogans. Writing only that a sprinter obeys the third law gains little. You must name the action force (push back and down on the blocks), the reaction force (ground pushes forward and up) and the result (the body accelerates out of the blocks). Always translate the law into the specific forces acting in the pictured skill.
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, , so .
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 N and kg.
Step 3: calculate the acceleration
Substitute: 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 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 2021-style6 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.Show worked answer →
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 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 2023-style4 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.Show worked answer →
A 4 mark answer needs the calculation and an interpretation using the second law.
- Calculation
- Rearranging gives 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, 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.