Forces · Classify
Non-contact forces
Every force so far has needed something to touch something. Three of them do not, and one of those is holding you to the planet right now.
Start here
The hair moves before the balloon arrives.
Rub a balloon on a jumper and bring it slowly towards someone's head. The hair stands up and reaches out to meet it — while there is still a clear centimetre of air in between.
What is pulling the hair?
A force is acting across the gap. Rubbing moved electric charge from the jumper to the balloon, and charge attracts and repels other charge without any contact at all. There are three forces in KS3 that work like this — gravity, magnetism and electrostatic force — and calling them non-contact forces is the whole of the classification.
A contact force needs the two objects to be touching: friction, air resistance, tension in a rope, the push of a table. A non-contact force acts across a gap with nothing in between: gravity, magnetism, and the electrostatic force between charges. Both kinds are measured in newtons and both change motion in exactly the same three ways.
At the bench · the sorter
Eight situations. Four labels.
0 of 8 labelled
Pick a situation, look at whether the two things are touching, then give it a label. The diagram marks the gap or the contact for you; the naming is yours.
Give it a label
Your label
—
The force
—
Contact or not
—
Pull or push
—
Unlabelled. Nothing is revealed until you commit to one of the four.
The three, and how to tell them apart
Only one of them can never push.
Gravity
Acts between anything with mass — you and the Earth, the Earth and the Moon, you and this page. It never switches off, and it gets weaker the further apart the two things are.
Tell it by: it only ever pulls. There is no such thing as gravitational repulsion.
Magnetism
Acts between magnets, and between a magnet and a magnetic material — iron, steel, nickel, cobalt. Not aluminium, not copper, not gold, so a magnet does not attract all metals.
Tell it by: like poles repel, unlike poles attract. It can push as well as pull.
The electrostatic force
Acts between electric charges, which is what rubbing two materials together moves about. Same reason your jumper crackles and cling film sticks to itself.
Tell it by: like charges repel, opposite charges attract. It fades as the charge leaks away.
That difference matters more than it looks. Magnetism and electrostatic force can cancel themselves out, because for every attraction there is a repulsion available. Gravity cannot, so it never cancels — it only ever adds up. Which is why the weakest of the three is the one that shapes planets, stars and galaxies.
Key fact
Gravity, magnetism and the electrostatic force act at a distance, with no contact and nothing needed in between. Gravity only ever attracts; the other two attract and repel. All are measured in newtons, and all change motion in the same three ways as a push or a pull.
Think again
“Something must be in between to carry the force across.”
It is a reasonable demand — every force you can feel with your hands is delivered by contact — and for two hundred years it bothered the best physicists alive. Newton himself would not claim to explain how the Sun pulls on the Earth across a hundred and fifty million kilometres of nothing; he only gave the arithmetic. Take the air out of a jar and a magnet still attracts a paperclip inside it; put a vacuum between the Earth and the Moon, which is what is actually there, and the pull continues. The modern answer is not a substance but a field: the magnet changes the space around it, and anything magnetic that enters that space feels a force. What travels is a change in the field, and it travels through nothing quite happily.
“There is no gravity in space.”
If that were true the space station would fly off in a straight line and the Moon would leave. The station orbits about four hundred kilometres up, where the Earth's pull is only slightly weaker than it is in your classroom, and the astronauts inside are not weightless — they are falling. So is the station, at exactly the same rate, which is why nothing inside presses on anything: they fall together, and the sideways speed means the fall keeps missing the planet. The word for what they experience is free fall, and you get a tiny sample of it at the top of a trampoline bounce.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Classify
Which of these is a non-contact force?
Rung 2 · The one that catches people
Astronauts float around inside the space station. Why?
Rung 3 · Explain
A plastic ruler rubbed on a sleeve picks up small pieces of paper from the desk before it reaches them. Explain why this is a non-contact force, and describe one thing you could do to show the force is real rather than the paper being blown about.
Rung 4 · Take it somewhere new
A maglev train floats about a centimetre above its track and is pushed along without anything touching it. Sort the forces acting on it into contact and non-contact, and explain why it can reach higher speeds than a train on steel wheels.
Key note
Most forces need contact, but three do not. Gravity acts between anything with mass and only ever attracts. Magnetism acts between magnets and magnetic materials, and can attract or repel. The electrostatic force acts between electric charges, and can also do both. All three act across a gap with nothing in between, all are measured in newtons, and all change motion in exactly the same ways as a contact push or pull.
Going further
Here is a comparison worth sitting with. Hold a small fridge magnet above a steel paperclip lying on a table, and the paperclip jumps up to the magnet. In that moment a magnet you could hide under a stamp has beaten the gravitational pull of the entire planet Earth on the same object. Gravity is by far the weakest of the three non-contact forces — absurdly, almost embarrassingly weak — and yet it is the one that decides the shape of the universe. The reason is the thing you met in the cards above: electric charges come in two kinds that cancel, and magnets have two poles that cancel, so on any large scale their effects add up to almost nothing. Mass has no opposite. Every kilogram of the Earth pulls in the same direction as every other kilogram, so gravity is the only one of the three that keeps accumulating all the way up to a galaxy.
Non-contact forces also make possible machines that touch nothing at all. A maglev train is held a centimetre clear of its track by magnetic repulsion and pushed along by magnetic attraction, so the only resistance left is the air — no wheels, no rails rubbing, no bearings, almost no friction. Induction hobs heat a pan across a gap; wireless chargers move energy into a phone without a plug; the read head of a hard disk flies above the surface without touching it. In each case the engineering payoff is the same: nothing that touches, nothing that wears out.
Before this lesson
Connects to
At GCSE this becomes
- Gravitational, magnetic and electric fields, field lines and field strength, and how the force between two objects falls with distance.
Where to next
Ask Mr Badmus AI
Got a situation you cannot decide how to label?
The sorter uses the three non-contact forces named at Key Stage 3. There are others in physics — the forces inside an atomic nucleus among them — and they are not in scope here. Air resistance is filed as a contact force because the air particles strike the surface; the diagram draws the two objects at a fixed size and does not show relative distances to scale.
Lesson content © MrBadmusAI.