Newton's three laws of motion

Every sporting movement obeys Newton's laws. Learn each law and a sporting example:

• 1st law — inertia: a body stays at rest, or moving at constant velocity, unless acted on by an external force. A stationary football sits still until a boot applies a force.
• 2nd law — acceleration: the rate of change of momentum is proportional to the force applied, so F = ma. A larger force on the ball produces a larger acceleration.
• 3rd law — action–reaction: for every action there is an equal and opposite reaction. A sprinter drives down and back into the blocks; the ground pushes them up and forward (the ground reaction force).

Exam tip: always pair the law with a specific example. State the law, name the force, then describe the effect on the body or implement.

Name the law

Force and Newton's second law

A force is a push or pull that can change an object's speed, direction or shape. Newton's second law gives us the equation for the size of that force:

Because force is proportional to the rate of change of momentum, a bigger force on a fixed mass produces a bigger acceleration. This is why a stronger athlete can accelerate the same shot put faster.

Units: one newton is the force needed to accelerate a 1 kg mass at 1 m/s². Multiply mass in kg by acceleration in m/s² to get force in N.

Your turn — resultant force

Momentum and impulse

Momentum is the quantity of motion a body has — the product of its mass and velocity:

Impulse is force × time, and it equals the change in momentum. In sport, applying a force for longer (a bigger follow-through) creates a bigger impulse and so a greater change in velocity.

In practice: a sprinter's foot contact shows a positive impulse (drive phase) followed by a smaller negative impulse (braking). A net positive impulse accelerates the runner.

Your turn — momentum

Why follow through?

Levers in the body

A lever has three parts: the fulcrum (F, the joint/pivot), the effort (E, the muscle force) and the load / resistance (R, the weight moved). Their order defines the class:

• 1st class — Fulcrum in the middle (E–F–R). Example: extending the neck by nodding at the atlanto-occipital joint; triceps extending the elbow.
• 2nd class — Resistance/load in the middle (F–R–E). Example: plantar-flexion when rising onto the toes. High mechanical advantage — moves large loads.
• 3rd class — Effort in the middle (F–E–R). Example: elbow flexion by the biceps. The most common lever in the body; favours speed and range of movement.

Memory aid: the middle letter of 1-2-3 maps to F-R-E (Fulcrum, Resistance, Effort). Say "1-2-3, F-R-E".

Mechanical advantage

Mechanical advantage (MA) tells you what the lever is good at — moving big loads, or moving fast over a big range:

• MA > 1 — the effort arm is longer; a small effort moves a large load. Second-class levers (rising onto the toes) work this way.
• MA < 1 — the resistance arm is longer; more effort is needed, but the load moves fast over a large range. Third-class levers (elbow flexion) work this way.

Trade-off: you cannot have both high force and high speed from one lever. Second-class levers favour force; third-class levers favour speed and range.

Which lever class?

Tap a statement, then tap the lever class it belongs to.

Your turn — mechanical advantage

Speed vs force

Projectile motion

A projectile (a shot, a long-jumper, a football) follows a curved parabolic flight path. We split its release velocity into two independent parts:

Three factors shape the flight path: the speed of release, the angle of release, and the height of release.

Your turn — vertical component

Angular motion & the spinning skater

Rotating bodies (a somersaulting gymnast, a spinning skater) obey their own rules:

• Moment of inertia — the resistance of a body to changing its state of rotation. It depends on mass and, crucially, how that mass is distributed relative to the axis. Mass spread far out = large moment of inertia.
• Angular momentum = moment of inertia × angular velocity. With no external torque, angular momentum is conserved.

So a skater who pulls the arms in reduces their moment of inertia; to keep angular momentum constant their angular velocity rises and they spin faster. Opening out again slows the spin.

Key idea: conservation of angular momentum lets performers control spin speed simply by changing body shape.

Faster spin

Fluid mechanics: drag, lift & spin

Moving through air or water, a body meets fluid forces:

• Drag — the resistance opposing motion through a fluid. It is reduced by streamlining, a smooth surface, and a small frontal area (the cyclist's tuck).
• Lift & the Bernoulli principle — faster-moving fluid exerts lower pressure. Shaping air flow so it travels faster over one side creates a pressure difference and a lift force (the discus, the aerofoil).
• The Magnus effect — a spinning ball drags air around with it, making flow faster on one side. The pressure difference swerves the flight: topspin makes a ball dip, backspin makes it float.

Same physics, opposite goals: a sprint cyclist wants to minimise drag; a swimmer's hand and a spin bowler exploit lift and the Magnus effect.

Match each term to its meaning

Tap a term on the left, then its matching definition on the right.

The big ideas to know

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