Newton's three laws of motion

Newton's laws underpin every technique. Learn the law and match it to a movement:

• 1st law — law of inertia: a body remains at rest or moving uniformly until a net external force acts. A curling stone glides in a straight line until friction or a sweeper's action changes it.
• 2nd law — law of acceleration: acceleration is proportional to the force and inversely proportional to mass, giving F = ma (force is proportional to the rate of change of momentum). A harder golf strike accelerates the ball more.
• 3rd law — law of reaction: every action has an equal and opposite reaction. A basketball player pushes down into the floor to jump; the floor pushes up with an equal reaction force.

Exam tip: examiners want the law named, the force identified and the sporting effect described — three marks, three parts.

Name the law

Force and Newton's second law

A force is a push or pull measured in newtons (N) that alters an object's motion. Its size is fixed by Newton's second law:

Because force is proportional to the rate of change of momentum, the same mass accelerates more when a larger net force acts. A heavier player therefore needs more force to reach the same acceleration.

Net force: only the resultant (net) force accelerates a body. Balanced forces (equal and opposite) produce no acceleration.

Your turn — resultant force

Momentum and impulse

Linear momentum is mass multiplied by velocity — how hard a moving body is to stop:

Impulse equals force multiplied by the time it acts, and it produces the change in momentum. Coaches use impulse two ways: increase it to speed up (long drive phase), or spread it over more time to cushion a landing and cut the peak force.

Impulse–momentum graphs: for a runner's foot strike, area above the time axis is positive (accelerating) impulse; area below is negative (braking) impulse. Net positive = speeding up.

Your turn — momentum

Softer landings

Lever systems in the body

Every lever has a fulcrum (F, the pivot/joint), an effort (E, the muscle force) and a load/resistance (R, the weight). The middle component names the class:

• First class — Fulcrum central (E–F–R). Example: the triceps extending the elbow; nodding the head at the atlanto-occipital joint.
• Second class — Resistance central (F–R–E). Example: standing on tip-toe (plantar-flexion) — the ball of the foot is the fulcrum, body weight the load, calf muscles the effort. High force output.
• Third class — Effort central (F–E–R). Example: the biceps flexing the elbow in a dumbbell curl. This is the body's most common lever and favours speed and range.

Recall trick: the numbers 1-2-3 point to the middle component F-R-E. First = Fulcrum, second = Resistance, third = Effort.

Mechanical advantage

Mechanical advantage (MA) is the ratio of the two lever arms, and it decides whether a lever is built for strength or for speed:

• MA > 1 — effort arm longer than resistance arm; a small effort overcomes a large load (second-class levers, tip-toe raise).
• MA < 1 — resistance arm longer; more muscular effort is needed, but the end of the lever moves quickly over a wide range (third-class levers, elbow flexion in a throw).

Design trade-off: long limbs give a big resistance arm (low MA) but high end-point speed — useful for throwers and strikers who want racket or club-head velocity.

Which lever system?

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

Your turn — mechanical advantage

Built for force

Projectile motion

A launched body — a javelin, a diver, a basketball — follows a parabola. We treat its motion as two independent components of the release velocity:

Three release factors govern the flight path: the speed, the angle, and the height of release. A high release point lets a lower, flatter angle still maximise distance.

Your turn — horizontal component

Angular motion & conserving spin

Rotating athletes — a diver's tuck, a trampolinist's twist — obey the rules of angular motion:

• Moment of inertia — the resistance of a rotating body to a change in its angular motion. It grows when mass is spread further from the axis (a layout position) and shrinks when mass is tucked in close.
• Angular momentum = moment of inertia × angular velocity. Once airborne, with no external torque, angular momentum is conserved.

So a diver leaves the board with fixed angular momentum, then tucks to cut the moment of inertia; angular velocity rises and they somersault quickly. Opening out before entry raises the moment of inertia and slows the rotation for a clean entry.

Key phrase: "moment of inertia down, angular velocity up" — the conservation of angular momentum in one line.

Quicker somersault

Fluid mechanics: drag, lift & spin

Objects moving through air or water experience fluid forces that a smart performer manages:

• Drag — resistance acting against motion through a fluid. A skier in a tuck or a swimmer holding a streamlined line reduces frontal area and surface friction to cut drag.
• Lift & the Bernoulli principle — where a fluid flows faster, its pressure is lower. Angling a discus or ski-jumper's body so air moves faster over the top creates a lower pressure above and an upward lift force.
• The Magnus effect — a spinning ball speeds the air on one side and slows it on the other, so pressure differs across the ball and its path curves. A footballer's sidespin bends a free kick around the wall.

Two sides of one coin: sprint and downhill athletes fight to reduce drag, while throwers and ball players use lift and the Magnus effect to gain distance or swerve.

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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