Contact and Non-Contact Forces: KS3 Physics Explained

A force is a push or pull that can change the shape, speed, or direction of an object. At KS3, forces are split into two groups — contact forces, which require physical touching, and non-contact forces, which act across empty space without any touching at all. Knowing which type you are dealing with is the first step in every mechanics problem.

What is a contact force?

A contact force only acts when two objects are physically touching. Remove the contact and the force immediately drops to zero.

Contact force | Description | Everyday example
Friction | Opposes motion between surfaces | Brakes slowing a bicycle
Normal (reaction) force | Pushes back perpendicular to a surface | Floor pushing up on your feet
Tension | Pulls along a rope, string, or cable | Cable holding an elevator
Drag (air resistance / water resistance) | Friction from a fluid opposing motion | Parachute slowing a skydiver
Upthrust (buoyancy) | Upward push from a fluid on a submerged object | Water holding a boat afloat
Compression | Squeezing force along a solid object | Spring being pushed together

Predict: if you push a heavy box across a rough floor, how many contact forces act on it? At least three — your applied push, friction opposing the motion, and the normal force from the floor acting upwards.

What is a non-contact force?

A non-contact force acts between objects that are not touching. It can operate across a vacuum — which is exactly why the Sun's gravity holds the Earth in orbit across 150 million kilometres of empty space.

Non-contact force | Acts between | Can attract or repel?
Gravitational | Any two objects with mass | Attract only
Magnetic | Magnetic materials and poles | Both (like poles repel, unlike attract)
Electrostatic | Charged objects | Both (like charges repel, unlike attract)

One of the most surprising things about non-contact forces is that they weaken with distance but never truly reach zero — gravity from Earth reaches across the entire universe, just becoming vanishingly small.

How do we represent forces using diagrams?

Forces are always shown as arrows on a free body diagram. The rules are:

The arrow starts at the object.

The arrow points in the direction of the force.

The length of the arrow is proportional to the magnitude (size) of the force.

Each arrow is labelled with the force name and size in newtons (N).

If arrows are balanced (equal in size, opposite in direction), the forces are balanced and the object does not accelerate. If one arrow is longer, the forces are unbalanced and the object accelerates in the direction of the resultant.

What is the resultant force?

The resultant (or net) force is the single force that has the same effect as all the individual forces acting together.

Worked example: A car engine exerts a driving force of 3 000 N forwards. Friction and air resistance together produce 1 200 N backwards. What is the resultant force?

Both forces act along the same line (but opposite directions).

Resultant = 3 000 − 1 200 = 1 800 N forwards.

The car accelerates forward.

If driving force equalled friction exactly (both 3 000 N), resultant = 0 N and the car travels at constant velocity (Newton's First Law).

How do gravitational and magnetic forces compare?

Both gravity and magnetism are non-contact forces, but they behave quite differently:

Feature | Gravity | Magnetism
What objects? | Any object with mass | Magnetic materials (iron, nickel, cobalt)
Direction | Always attractive | Can attract or repel
Depends on | Mass and distance | Magnetic strength and distance
Can be shielded? | Not practically | Yes, with mu-metal or thick iron
Acts in a vacuum? | Yes | Yes

An important prediction to test: place a plastic ruler near a magnet — is it attracted? No. Plastic has no magnetic properties. Now place an iron nail — it jumps towards the magnet. The difference is all about what the material is made of, not just its mass.

What is friction and why does it matter?

Friction is the contact force that opposes the relative motion (or tendency to move) between two surfaces in contact. It acts parallel to the surface and opposite to the direction of motion.

Friction is not always unhelpful:

Useful friction: Car tyres gripping the road, shoe soles, writing with a pen, brakes on a bike.

Unhelpful friction: Engine parts wearing down, dragging a heavy sofa across a floor, air resistance slowing an aircraft.

Engineers choose surface materials and lubricants (oil, grease) to maximise useful friction where they need grip and minimise it where they need efficiency.

What is electrostatic force at KS3?

When certain materials (like a balloon or plastic comb) are rubbed, they gain a static electric charge by gaining or losing electrons. Two objects with opposite charges attract; two objects with the same charge repel. This is the electrostatic non-contact force.

Predict: if you rub a balloon on your hair and hold it near tiny shreds of paper, what will happen? The charged balloon induces an opposite charge on the paper's surface and attracts them — even though the paper was originally neutral.

Frequently asked questions

What is the difference between mass and weight?

Mass is the amount of matter in an object (measured in kg) and does not change wherever you are in the universe. Weight is the gravitational force acting on that mass (measured in newtons, N) and depends on the local gravitational field strength: Weight = mass × g. On the Moon (g ≈ 1.6 N/kg) you weigh about one-sixth of your Earth weight, but your mass is identical.

Can a non-contact force do work?

Yes. Work is done whenever a force causes displacement in the direction of the force. Gravity does work on a falling object (that is how GPE converts to KE), and a magnetic force does work when it pulls an iron nail across a table. The force does not need physical contact to transfer energy.

Why do like poles of a magnet repel?

Magnetic poles arise from the collective alignment of tiny magnetic domains within a material. When two north poles (or two south poles) are brought together, their magnetic fields interact and push against each other — the field lines try to avoid overlapping in the same direction. Unlike poles (north–south) have field lines that run from one to the other, so they attract.

Is upthrust a contact or non-contact force?

Upthrust (buoyancy) is a contact force. It arises from water molecules colliding with and pushing on the submerged surface of an object. Remove the water and the force instantly vanishes — just as any contact force should.

Curious to predict what happens next in a forces experiment? Book a free session with Professor Newton at aitutors.me and discover how to think like a physicist.