Speed tells you how fast

An object that is moving covers distance as time passes. Speed tells you how much distance it covers each second:

Everyday speeds are often given in metres per second (m/s) or kilometres per hour (km/h). A brisk walk is about 1.5 m/s; a fast sprinter reaches about 10 m/s.

Average vs instantaneous: most journeys speed up and slow down, so the equation gives the average speed over the whole trip — not necessarily the speed at any single moment.

Using speed = distance ÷ time

Rearranging: the same triangle gives you the others — distance = speed × time, and time = distance ÷ speed. Cover the one you want to find.

Your turn — average speed

Your turn — rearranging

Drawing a journey as a graph

A distance–time graph shows how far an object is from its start at every moment. Time goes across the bottom; distance goes up the side.

Key idea: a straight sloping line means a steady (constant) speed. A horizontal line means the object is not moving — the distance isn't changing.

Reading the graph

Which line is faster?

Speed depends on your point of view

How fast something seems to move depends on what you compare it to. This is relative motion.

To a passenger on a moving train, someone walking down the aisle seems slow — but to a person standing on the platform they are moving at about the train's speed.

Passing trains

Why doesn't a car with balanced forces stop?

Forces are pushes and pulls

A force is a push or a pull. A force always comes from an interaction between two objects — one object pushes or pulls another.

• Your hand pushes a door — hand and door interact.
• A magnet pulls a paperclip — magnet and clip interact.
• The Earth pulls you down (gravity/weight) — you and the Earth interact.

Watch out: a single object on its own can't have a force "in" it. Forces always involve two objects interacting. We measure force in newtons (N) using a newtonmeter (a spring scale).

Two families of force

Forces come in two families, depending on whether the objects must touch:

• Contact forces — the objects touch: friction, air resistance, the push of a table (normal contact force), tension in a rope, applied pushes and pulls.
• Non-contact forces — act at a distance, with no touching: gravity (weight), magnetism, electrostatic force.

Contact or non-contact?

Tap a force, then tap the box it belongs in.

Stretching and squashing

Forces can change an object's shape. Pull on a spring and it stretches. Push on it and it squashes (it is compressed).

Hang a weight on a spring and it stretches until the spring's upward pull equals the weight. Then it hangs still. A table works the same way: it squashes very slightly under a book and pushes back up.

Extension is how much longer the spring gets: extension = stretched length − original length. It is not the new length.

Force and extension

Hang weights on a spring one at a time. Measure the extension each time, then plot force against extension.

For a spring, you get a straight line through the origin: the extension is proportional to the force. Double the force and the extension doubles. If 2 N stretches it 3 cm, 4 N stretches it 6 cm. This is Hooke's law.

Only up to a point: Hooke's law is a special case. Past its limit of proportionality, a spring's line curves. A rubber band's graph, for example, is not a straight line.

Stretching takes work

To stretch or squash a spring, your force must move through a distance. When a force moves something, work is done, and doing work transfers energy.

Here the energy goes into the spring's elastic store. The further you stretch it, the more work you do and the more energy it stores. Let go, and the spring can transfer that energy again, for example to launch a toy.

Watch out: a stretched spring stores energy, not force.

Double the force

When forces balance

Objects usually have several forces on them at once. We show each force as an arrow: the longer the arrow, the bigger the force.

Big idea: when forces are balanced (equal and opposite), they cancel out. The object either stays still OR keeps moving at a steady speed in a straight line. When forces are unbalanced, the motion changes — it speeds up, slows down, or changes direction.

Balanced does not mean stopped

Lots of people think "balanced forces" always means "not moving". That's only half true.

The truth: balanced forces mean the motion doesn't change. A car cruising at a steady 30 m/s on a flat road has balanced forces (driving force = friction + air resistance), yet it is definitely moving. Balanced forces = "no change in motion", which can mean stationary OR steady speed in a straight line.

So next time you see equal, opposite arrows, don't assume the object is still — it could be gliding along at a constant speed.

Balanced or unbalanced?

For each situation, tap whether the forces are balanced or unbalanced.

Cruising along

Why is a door handle never next to the hinges?

Forces that turn things — moments

A force can also make something rotate around a pivot. The turning effect of a force is called its moment.

Why door handles are on the edge: a handle far from the hinge gives a big distance, so even a small push makes a large moment — easy to open. Push near the hinge and it barely turns.

Using a spanner

Pressure spreads a force over an area

Pressure tells you how concentrated a force is — how much force pushes on each bit of area:

The same force over a smaller area gives a bigger pressure. That's why a sharp knife (tiny area) cuts, while lying on a bed of many nails (large total area) doesn't hurt.

Pressure in fluids: liquids and gases also push. Pressure in a fluid gets greater the deeper you go, and it pushes in all directions — that's why your ears hurt at the bottom of a swimming pool.

Your turn — pressure

The air above you pushes too

Air has weight. The air above us presses on everything, in all directions: this is atmospheric pressure. At sea level it is about 100 000 N/m².

Go higher and there is less air above you, so less weight of air pressing down. Atmospheric pressure decreases with height.

Real life: an empty plastic bottle sealed on a mountain top gets crushed on the way down. Lower down, the air pressure outside is greater than the pressure inside.

Deeper means more pressure

The deeper you go in a liquid, the more liquid there is above you, pressing down with its weight. So pressure in a liquid increases with depth.

The pressure pushes in all directions, and always at right angles to any surface it touches.

Real life: a dam is built thicker at the bottom, because the water pushes hardest there.

Why water pushes up

Put an object in water. The water pushes on it from every side, and the sideways pushes cancel out.

But the bottom of the object is deeper than the top, so the water pressure there is greater. The push up on the bottom is bigger than the push down on the top. The result is an upward force called upthrust.

Common mistake: an object in water does not lose weight. Its weight is the same; the upthrust pushes up against it.

Float or sink?

An object in water has two forces on it: its weight (down) and the upthrust (up).

• Upthrust equals weight: the forces are balanced, so the object floats.
• Weight is bigger than the upthrust, even when the object is completely under water: it sinks.

A floating object settles just deep enough for the upthrust to equal its weight.

Afloat

Why is a door handle never next to the hinges?

Your turn — average speed again

Where do forces come from?

The key ideas & equations

You've covered the KS3 physics of motion and forces. Press Finish to see your score.