Forces · Model
What a force is
Push a wall hard enough and you move backwards. Nothing about you got stronger — so what actually pushed you?
Start here
The wall pushed you.
Stand on a skateboard, put your hands on a wall and push. You roll away from the wall. The wall did not move, did not spend anything, and was not trying — and it is the only thing that could have pushed you.
So what is a force?
A force is a push or a pull, and it is never something one object has on its own. It takes two: you and the wall, measured in newtons. Name only one object and you have not finished describing the force.
Every force in this unit is one object pushing or pulling another. The job on this page is to get into the habit of naming both ends of it, and giving its size in newtons.
At the bench · the interaction board
Find the object on the other end
0 of 5 cases opened
Pick a case
A ball leaves a boot at speed. What is on the other end of that force?
The foot and the ball, touching for roughly a hundredth of a second. Each feels about 300 N, in opposite directions.
Air resistance is real, but it is not what sends the ball up the pitch. The object on the other end is the foot, and it feels about 300 N too.
Speed is not an object, so nothing can be on the other end of it. The foot is the second object, and it feels about 300 N too.
A caravan is dragged forward along a motorway. What is on the other end of that force?
The tow bar pulls the caravan forward with about 2 000 N, and the caravan pulls back along the bar with about 2 000 N. A pull, and the two objects are touching.
The road does push up on the tyres and rub backwards on them, but the pull along the line of travel comes from the tow bar, at about 2 000 N.
Weight is a force, not an object, and it acts downwards rather than forwards. The tow bar is the second object, at about 2 000 N.
A paperclip lifts off a desk and flies upwards. What is on the other end of that force?
The magnet pulls the paperclip up with about 2 N and the paperclip pulls the magnet down with about 2 N, across a gap of two centimetres with nothing in it.
The desk was holding the paperclip up, not pulling it upwards. The pull comes from the magnet, across the gap, at about 2 N.
Take the air out of the gap and the pull is unchanged. The second object is the magnet, at about 2 N.
A swimmer moves forward down the lane. What is on the other end of that force?
The hand pushes about 150 N of water backwards, and the water pushes the swimmer forwards with about 150 N. Watch the wake: that is the other half of the pair.
Muscles are part of the swimmer, and a force needs a second object. That object is the water, at about 150 N.
The rope marks the lane and is not touched. The forward push comes from the water, at about 150 N.
The Moon keeps curving around the Earth instead of leaving. What is on the other end of that force?
The Earth pulls the Moon with about 200 billion billion N, and the Moon pulls the Earth with the same force, across 384 000 km of empty space.
Sunlight does push on things, but nowhere near hard enough to hold a moon. The Earth is the second object, at about 200 billion billion N.
Speed is not an object, and speed on its own would send the Moon off in a straight line. The pull comes from the Earth, at about 200 billion billion N.
The two objects
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Each force
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Push or pull
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Every force, three questions
Answer all three or you have not described it.
Which two objects?
Name both. “A force on the caravan” is half an answer; “the tow bar pulls the caravan” is a whole one.
Push or pull, and how big?
Forces are measured in newtons, written N. An apple resting on your hand presses down with about 1 N.
Touching, or across a gap?
Most forces need contact. Gravity, magnetism and static electricity do not, and they are no less real for it.
Key fact
A force is a push or a pull on one object, caused by a second object. Forces are measured in newtons (N), and they always come in pairs of the same size acting on the two different objects.
Think again
“A moving object has force in it, and the force runs out.”
This is the oldest idea in physics and almost everyone arrives holding it: a thrown ball carries a supply of force that gradually empties, which is why it slows down and falls. It is wrong in a way that matters, because a force is not stuff and cannot be stored. A ball has speed, and it has energy — both of which really are properties of the ball on its own — but the force on it exists only while some second object is pushing or pulling it. The moment your hand lets go, the hand's push on the ball stops existing. What slows the ball down is not the push running out; it is two other objects, the air and the Earth, pushing and pulling on it the whole way.
“A table is not doing anything. It is just there.”
Put a book on your palm and you can feel yourself pushing up; the table is doing the same thing and feels nothing because it is not alive. Every surface presses back on whatever presses into it — that is what stops the book going through the table, and it is why a shelf can only take so much before it snaps. A force does not require effort, intention or a living thing at either end. It requires two objects.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Name the pair
A footballer heads a ball. Which statement describes the force correctly?
Rung 2 · The one that catches people
A magnet lifts a paperclip across a gap of two centimetres. Which statement is right?
Rung 3 · Explain
Someone on a skateboard pushes a wall and rolls backwards. Name every object involved and every force in the pair, and say which force moved the skater.
Rung 4 · Take it somewhere new
A spacecraft far from any planet fires its engine and speeds up. There is nothing around it to push against. Name the second object in that interaction, and explain how you know a second object must exist.
Key note
A force is a push or a pull, measured in newtons. It is not something an object owns and not something that runs out — it exists only while two objects are interacting, and it acts on both of them equally and in opposite directions. Some of those interactions need contact and some do not.
Going further
If the two forces in every pair are always equal and opposite, why does anything ever move? Because they act on different objects, and only forces on the same object can cancel. When you push the wall, the wall's push acts on you and yours acts on the wall — nothing cancels anything. You accelerate because your total force is not zero, and the wall does not because it is bolted to a building, and to the Earth, which is very hard to shift with 200 N.
A rocket in deep space has nothing to push against, and it accelerates anyway. The second object is the exhaust: the engine throws several tonnes of hot gas backwards every second, and the gas pushes the rocket forwards with exactly the same force. This is why a rocket works better in a vacuum than in air, which is the opposite of what most people guess. It is also why the newton is defined the way it is — one newton is the force that changes the speed of one kilogram by one metre per second, every second, and nothing in that definition mentions anything touching.
Before this lesson
- Nothing — this is where the unit starts.
Connects to
At GCSE this becomes
- Contact and non-contact forces, free-body diagrams, and Newton's third law as interaction pairs.
Where to next
- Next: Drawing and adding forces
- Previous: Relative motion
Describing motion
Ask Mr Badmus AI
Stuck naming the second object in a case of your own?
Force sizes on the interaction board are typical values, rounded, and are marked “about” because they depend on how hard, how fast and how far apart. The board draws one interaction at a time and leaves out every other force acting on the two objects.
Lesson content © MrBadmusAI.