KS3 · Physics

What forces do

Kick a football and it rolls on after your foot has gone. So what's pushing it forwards? Nothing is — and that one idea changes how you see every force.

Forces · Four scenes

What's pushing — and what's changing?
footforce of the footon the ball

View: 1 · The kick. Showing 2 layers: Ground, The kick

View

four scenes — spot the arrows ↑

The foot pushes the ball. That push is a force — the force of the foot on the ball. Two objects are involved: the foot provides the force, and the ball is what it acts on. And watch what it does: the ball goes from still to moving, so the force has changed the ball's speed.

The solid blue arrows are the forces we're tracking, each named 'the force of A on B'. In every scene, ask what that force changes. The dashed grey arrow only shows which way something is moving — it is not a force.

What do you really think?

Why does the ball keep rolling?

You kick a football across the park. It rolls on for a while after your foot has left it, slows down, and eventually stops.

Which of these is closest to what you think is going on?
How sure are you?

Speed, direction, shape

What does each force change?

A force can change three things about an object: its speed (speeding it up or slowing it down), the direction it's moving in, or its shape. For each event, tick every box for something that changes. Some events might change nothing at all.

Squashing a balloon between your hands
Stretching a spring by pulling on its end
A ball rolling along the ground, slowing to a stop
Pushing a still trolley so it starts to move
Pushing a trolley round a corner at a steady speed
A cyclist riding at a steady speed in a straight line

Predict, then check

Three trolleys are rolling towards you, and you have to stop one of them.

Which trolley needs the greatest force to stop it?

Your turn to explain

The steady cyclist

A cyclist is riding along a flat, straight road at a steady speed. There is a forward force on the bicycle, but the bicycle is not speeding up. Explain why. Name one of the forces using the words 'the force of … on …'. [3 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Forces don't keep things moving. They change how things move — or change their shape.

What you need to know

  • A force is a push or a pull by one object on another. Every force involves two objects: one provides the force and the other is what it acts on. Name it as 'the force of A on B'. If nothing is pushing or pulling an object, no force acts on it.
  • A force is not something an object has. A moving object doesn't carry a force that keeps it going and then runs out. Forces come from other objects, and a force is not energy or power.
  • A force is needed to change an object's speed, its direction of motion or its shape. Forces change speed; they don't give it. An object slows down when forces act opposite to its motion.
  • If a force acts but causes no change, other forces must be acting and cancelling out its effect — like two equal pushes from opposite sides, or air resistance on a cyclist moving at a steady speed.
  • Heavier objects are harder to start, stop or turn, and faster objects are harder to stop. Each takes a greater force. It also takes a greater force to throw something faster, further or higher.

The big picture

A force is a push or a pull by one object on another, so we can name it 'the force of A on B'. Objects don't carry forces around: a rolling ball has no forward force on it, and a force is not energy. Forces change an object's speed, direction or shape — they change speed rather than give it. If a force acts but nothing changes, other forces are cancelling it. Heavier or faster objects need a greater force to stop them.

Key points

1Force = a push or a pull of one object on another.
2Name it: 'the force of A on B'. No A pushing or pulling? No force.
3Objects don't carry forces around with them.
4A force can change speed, direction or shape.
5No change even though a force acts? Other forces are cancelling it.
6More mass, or more speed, means a greater force is needed to stop it.

Worked example

Problem

A magnet is brought close to an iron nail lying on a table, and the nail slides across the table towards the magnet. Name the force that moved the nail in the form 'the force of A on B', and say what that force changed.

⚠ Watch out

Easy mistake: seeing a bike cruising at a steady speed, or a ball pushed equally hard from both sides, and deciding no forces act on it. The forces are there — they're cancelling each other out. 'Nothing is changing' doesn't have to mean 'no forces'.

🧠

Memory hook

Forces are CHANGERS, not KEEPERS. For anything, ask two questions: What's pushing or pulling on it — and what's changing? If nothing's changing, go looking for the force that cancels.

✓

Check yourself

Cover the page. Name the three things a force can change about an object. Then explain why a football rolling along after a kick has no forward force on it.

Flashcards

(13)
What is a force?
A push or a pull that one object exerts on another.
How many objects are involved in every force?
Two: one that provides the force, and one that the force acts on.
How do you describe a force precisely?
As 'the force of object A on object B' — for example, the force of the foot on the football.
Nothing is pushing or pulling on an object in a certain direction. What force acts on it in that direction?
None. A force always comes from another object pushing or pulling.
Does a moving object carry a force that keeps it moving?
No. A force isn't something an object has — forces come from other objects.
Is a force a kind of energy?
No. A force is a push or a pull. It is not energy, and it is not power.
What three things can a force change about an object?
Its speed, its direction of motion, and its shape.
Do forces give objects speed?
No — forces change speed. They can speed an object up or slow it down.
Why does a moving object slow down?
Because forces act on it in the opposite direction to its motion.
A force acts on an object, but nothing changes. What must be happening?
Other forces are acting on it and cancelling out the first force's effect.
Which is harder to push round a corner: a full trolley or an empty one?
The full one. An object with more mass needs a greater force to change its direction.
Which needs a greater force to stop: a fast-moving object or a slow one?
The fast one. The motion of faster objects is harder to stop.
What does it take to throw a ball faster, further or higher?
A greater force on the ball.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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