Contact and non-contact forces

Forces always come from one object interacting with another. Eduqas splits them into two families:

• Contact forces — the objects have to touch: friction, the normal contact (support) force, tension, air resistance.
• Non-contact forces — they act at a distance, through a field: gravity, magnetism, electrostatics.

Forces are vectors: each one has a size and a direction, so we always show forces as arrows — long arrow = big force, pointing the way the force acts.

Contact or not?

Weight = mass × gravitational field strength

Weight is the gravitational force pulling an object down. It depends on the object's mass and the strength of the gravitational field it is in:

Mass vs weight: mass (kg) is the amount of "stuff" and never changes. Weight (N) is a force and changes if g changes — you'd weigh less on the Moon (g ≈ 1.6 N/kg) but your mass is the same. Use g = 9.8 N/kg on Earth.

Your turn — weight

Resultant force & free-body diagrams

A free-body diagram shows just one object with every force on it drawn as an arrow from the object's centre. Add the forces (taking direction into account) to get the single resultant force.

Balanced ≠ stationary. A zero resultant means no change of motion — the object stays still or keeps moving steadily. Balanced just means the arrows cancel out.

Reading the arrows

Resolving forces by scale drawing

When forces act at angles, you can find the resultant by drawing the arrows to scale, tip-to-tail, and measuring the closing arrow. The reverse — splitting one force into two perpendicular parts — is called resolving.

Higher tier only. You only need scale drawings (and resolving into right-angle components) on the Higher paper — Foundation does not assess this.

Hooke's law: force & extension

It takes more than one force to stretch, bend or compress an object. For a spring (up to its limit of proportionality) the extension is directly proportional to the force:

Elastic vs inelastic: an elastic distortion springs back to its original shape; an inelastic (plastic) distortion does not. x is the extension (how much longer it gets), not the total length. Required practical SP3.1: hang masses on a spring and plot the force–extension graph.

Your turn — Hooke's law

Pressure = force ÷ area

In a fluid (a liquid or a gas) the pressure pushes normal (at 90°) to every surface it touches. Pressure tells you how concentrated a force is over an area:

The same force over a smaller area gives a bigger pressure — which is why a sharp knife (tiny area) cuts so easily, while snowshoes (big area) stop you sinking.

Your turn — pressure

Pressure in a column of liquid

Dive deeper and the water above presses harder, so the pressure increases with depth. It also depends on the liquid's density — but not on the shape of the container:

Higher tier only. Liquid pressure depends on depth and density — not the container's width or shape. A narrow tube and a wide tank of the same depth have the same pressure at the bottom.

Your turn — pressure with depth

Upthrust, floating and sinking

Because liquid pressure is greater at the bottom of an object than at the top, there is an overall upward force called upthrust. Whether an object floats depends on how its weight compares with the upthrust:

Atmospheric pressure: our atmosphere is a deep "ocean" of air pressing on us. Climb a mountain and there is less air above you, so atmospheric pressure falls with height.

Float or sink?

Moment = force × perpendicular distance

A force that makes something turn about a pivot has a moment (a turning effect). It depends on the force and on how far that force acts from the pivot, measured perpendicular to the force:

Watch the distance. The moment uses the distance perpendicular to the force, not just any distance from the pivot. Push along the spanner (in line with the handle) and the perpendicular distance is zero — no turning at all.

Your turn — moment

The principle of moments

When something balances and does not turn, the turning effects cancel: the total clockwise moment equals the total anticlockwise moment about the pivot.

Levers and gears multiply force. A lever uses a long distance on one side to turn a big load with a small effort. Gears do the same with teeth: a larger gear turns with a bigger moment than a smaller one driven by the same force.

Your turn — balancing the seesaw

Contact or non-contact?

Tap a force, then tap the box it belongs in.

Pick the right equation

Tap the equation you'd use for each situation.

The equations to know

You've covered all three parts of Eduqas Topic 3 — forces & their interactions, pressure in fluids, and moments, levers & gears. Press Finish to see your score.