Contact and Non-Contact Forces

You already know a force as a push or a pull, and you know how several of them add up to a resultant force. Now for a tidy piece of bookkeeping that physicists use to keep every force straight: each and every force in the universe belongs to one of just two families.

A contact force only acts when two objects are touching. Let go, break the contact, and the force instantly vanishes.
A non-contact force reaches across a gap — it acts at a distance, through empty space, with nothing physically joining the two objects at all.

That second family sounds almost magical, and to early scientists it truly was baffling. How can the Earth pull an apple it isn't holding? How can a magnet grab a paperclip from a centimetre away? Yet both are everyday non-contact forces, and by the end of this page you'll be able to sort any force — weight, friction, tension, magnetism — into the right family and draw it on a proper force diagram.

Contact forces: something has to be touching

Most of the forces you meet in ordinary life are contact forces. They are the pushes and pulls that happen where two surfaces, or an object and a fluid, are pressed together. The moment the surfaces separate, the force is gone. Here is the usual cast, all worth knowing by name:

Friction — the grip between two touching surfaces that resists sliding. A book slid across a desk slows and stops because friction acts wherever the two surfaces meet.
Air resistance and drag — a fluid (air or water) pushing back on whatever moves through it. A cyclist, a parachute, a boat: all feel drag only while touching the fluid.
Tension — the pull carried along a stretched rope, string, cable or chain. A dog straining on its lead, a swing hanging from a beam, a bucket on a rope down a well.
The normal (reaction) contact force — a surface pushing back on whatever rests on it, always at right angles to the surface. It's what stops a book falling through the table.
Upthrust — the upward push of a liquid or gas on anything sitting in it, which is why a boat floats and a hot-air balloon rises.
An applied push or pull — your hand shoving a trolley, a foot kicking a ball, an engine's thrust driving a car forward.

Notice the pattern: for every one of these, you can put your finger on the exact spot where the two objects touch. No touching, no force. That single test — "are they in contact?" — is the whole definition.

Non-contact forces: acting across a gap

There are only three non-contact forces at GCSE, and it's worth memorising the short list, because everything not on it is a contact force:

The gravitational force (weight) — the Earth pulling every object towards its centre, whether or not anything is touching it. It even reaches out through space to hold the Moon in orbit.
The magnetic force — the pull or push between magnets, and the pull of a magnet on iron, steel, nickel or cobalt, across a gap of air.
The electrostatic force — the pull or push between electric charges. A charged balloon lifts your hair towards it; a comb rubbed on a jumper bends a thin stream of water without touching it.

So how does one object shove another it isn't even touching? The idea physicists use is a field: a region of space around an object where another object feels a force. A mass is surrounded by a gravitational field, a magnet by a magnetic field, a charge by an electric field. Bring a second object into that field and it feels the force — the field is the invisible reach of the first object stretching out into the gap. The stronger the source and the closer you are, the stronger the field, and the bigger the force.

Try it: build the force diagram

Pick a scene and watch its free-body diagram appear — the object shrunk to a simple shape, with one labelled arrow for every force acting on it. The arrows are colour-coded: one colour for contact forces, another for non-contact forces (the label says which too, so you never rely on colour alone). Step through all four and watch which family each force belongs to. The magnet is the surprising one — held near the fridge but not yet touching it, both its forces are non-contact, so it has no contact force at all.

Every diagram shows weight pulling down — gravity is a non-contact force that never switches off. The other arrow is what changes: the table's normal contact force, the drag on the skydiver, the tension in the lamp's wire, or the magnet's non-contact pull towards the fridge.

Drawing and reading free-body diagrams

A free-body diagram is how you identify and classify the forces on an object in one clean picture. The rules are few:

Shrink the object to a dot or a small box — you only care about the forces, not the shape.
Draw one arrow per force, starting at the object and pointing the way the force acts.
Make a longer arrow mean a bigger force, and label every arrow with its name.
Draw only the forces acting on the object — not the forces the object pushes back on other things.

Worked example — a book resting on a table. Two forces act. Its weight pulls straight down (non-contact, through the gravitational field). The table's normal contact force pushes straight up, at right angles to the surface (contact). They are equal and opposite, so the book stays balanced — but one is a contact force and one is not.

Worked example — a skydiver falling. Again weight pulls down (non-contact). This time the second force is air resistance pushing up (contact, because the diver is touching the air). Early in the fall the weight arrow is longer, so there is a downward resultant and the diver speeds up; later the arrows match and the fall steadies. Same two families, different lengths.

A force does not always need touching. The commonest slip is assuming every push or pull comes from contact. Gravity, magnetism and static electricity all act across empty space through a field — a dropped ball is pulled down with nothing above it, a paperclip leaps to a magnet through thin air.
Weight acts even when nothing is touching the object. A ball at the top of its throw, an astronaut coasting, a satellite in orbit — all still have weight, because the Earth's gravitational field reaches them. Non-contact means the force doesn't need contact, not that it switches off.
The normal contact force is perpendicular to the surface, not straight up. On a flat table it happens to point up; on a ramp it points at right angles to the slope, tilted away from vertical. "Normal" is a maths word meaning "at 90°", nothing to do with "ordinary".

Stretch a tape measure from here to the Moon and you'd unwind about 380,000 kilometres of empty, airless space — no rope, no rod, nothing joining it to the Earth. Yet our planet holds it looping around us, month after month, by pure gravity. That is a non-contact force reaching across a distance you could stack thirty Earths inside, all through the Earth's gravitational field thinning out into the void but never quite fading to nothing.

Magnets can perform the same trick against gravity itself. A maglev train carries no wheels touching the track — powerful magnets lift the whole carriage a few centimetres clear and shove it forward, so it glides on a cushion of pure magnetic field with nothing touching the rail. No contact, no friction, and speeds over 600 km/h. Non-contact forces aren't a curiosity; we build trains out of them.