9.1 Forces & Motion

9.1 Forces & Motion

A force is simply a push or a pull. You cannot see a force itself, but you can always see what it does: it can start something moving, stop it, speed it up, slow it down, change its direction or change its shape. Scientists measure forces in newtons (N), named after Sir Isaac Newton — a small apple weighs about 1 N, which is a handy mental picture whenever a question mentions newtons. Forces run right through the ICAS Science papers: the Introductory paper asks about the movement of objects in simple situations, and Papers G and H (around Years 9 and 10) tackle forces head-on, asking students to draw conclusions about forces in specific situations and to compare the forces acting on a body in air and in water.

Contact and Non-Contact Forces

Forces come in two families. Contact forces only act when two things are physically touching — your hand pushing a swing, or a shoe gripping the path. Non-contact forces act across empty space without any touching at all. Sorting a force into its family is a common early ICAS question, so learn the members:

That last one surprises people. A book resting on a desk does not crash through it because the desk pushes back up on the book — that upward push is the normal force. It always acts at right angles to the surface, which is what “normal” means in mathematics.

Notice that air resistance is a contact force: the object must be touching air for it to act. A skydiver feels drag precisely because they are colliding with air molecules the whole way down. Gravity and magnetic force, by contrast, work through empty space — the Earth pulls on the Moon without anything touching at all.

Balanced and Unbalanced Forces

Usually more than one force acts at once, and what happens next depends on how they add up.

• Balanced forces are equal in size and opposite in direction, so they cancel out. Nothing changes: an object at rest stays at rest, and an object already moving keeps moving at the same speed in a straight line. A book on a desk feels gravity pulling down and the normal force pushing up — equal and opposite — so the book sits still. A car cruising at a steady 60 km/h on a straight highway also has balanced forces: the engine's forward push matches the friction and air resistance pushing back.
• Unbalanced forces do not cancel, and the leftover force — called the net force — changes the motion. An unbalanced force is what makes things start moving, stop, speed up, slow down, change direction or change shape (squashing a ball of dough changes its shape without moving it anywhere).

When you kick a stationary soccer ball, your boot's force is unbalanced, so the ball starts moving. Air resistance and grass friction then push against it, so it slows and stops — another change caused by unbalanced forces.

Reading Force Diagrams

ICAS draws forces as arrows, and each arrow tells you two things at once: its direction shows which way the force acts, and its length shows how big the force is. A bigger arrow means a bigger force. So if a diagram of a falling parachutist shows a long downward arrow (gravity) and a short upward arrow (air resistance), the forces are unbalanced downwards and the parachutist speeds up. When the parachute opens, the upward arrow grows to match the downward one, the forces balance, and the fall settles to a steady speed. You can answer most force-diagram questions just by comparing arrow lengths.

Friction: Helper and Hindrance

Friction gets blamed for slowing things down, but life without it would be chaos. Helpful friction includes the grip between your shoes and the footpath (you would slip as if on ice without it), bicycle and car brakes rubbing on the wheel, and even a pencil leaving a mark on paper. Unhelpful friction wears out shoe soles and tyres, makes bicycles harder to pedal, and turns useful energy into wasted heat in engines and machinery. People reduce friction with lubricants such as oil and grease, with smooth or streamlined shapes, and with wheels — which brings us to movement itself.

Rolling, Sliding and Bouncing

The Introductory paper's favourite topic — how objects move in simple situations — really comes down to three patterns. Sliding means surfaces rub directly, so friction is large: a book pushed across carpet stops quickly. Rolling replaces sliding contact with rolling contact, so friction is much smaller: the same push sends a ball far across the same carpet. That is why wheels, from shopping trolleys to trucks, are such a useful invention. Bouncing happens when an object hits a surface and the collision force changes both its shape (the ball squashes) and its direction (it springs back up) — a neat demonstration of two effects of an unbalanced force in a single moment. When an ICAS stimulus shows a toy on a ramp or a ball on different surfaces, ask yourself which movement pattern is happening and which forces are helping or resisting it.