This mini-lesson walks you through the whole of Eduqas Topic 3 — Forces: contact and non-contact forces, weight, free-body diagrams, Hooke's law, pressure in fluids and moments, levers and gears.
Make a guess: are there really no forces on the stuck car?
Mission 1Are there no forces on the stuck car?
Mission 2Why does a steel ship float?
Mission 3Why does a longer spanner work better?
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
3.1 · Forces and their interactions
Contact and non-contact forces
Forces always come from one object interacting with another. We can sort 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.
Gravity, magnetism and electrostatics act without touching — through a field.
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.
Quick check
Contact or not?
?Two magnets repel each other across a small air gap, without touching. Which type of force is this?
3.1 · Weight
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:
W = m gweight (N) = mass (kg) × gravitational field strength (N/kg)
Mass vs weight:mass (kg) is the amount of "stuff" and is the same wherever you are. 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.
Worked example
A 5 kg bag on Earth (g = 9.8 N/kg).
W = m g = 5 × 9.8 = 49 N
Calculate
Your turn — weight
1A school bag has a mass of 7 kg. Using g = 9.8 N/kg, calculate its weight on Earth.
N
Hint: W = m g = 7 × 9.8.
3.1 · Free-body diagrams · Higher tier
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.
Up balances down (40 N = 40 N) and push balances friction (30 N = 30 N), so the resultant is zero.
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.
Quick check
Reading the arrows
?A sledge is pushed forwards with 50 N while friction pushes back with 50 N, and its weight is balanced by the ground. What is the resultant force on the sledge?
Mission 1 check
Are there no forces on the stuck car?
?In a tug of war, Team A pulls the rope left with 800 N. Team B pulls it right with 800 N. The rope does not move. Which statement is correct?
3.1 · Higher tier only
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.
Draw 4 N, then 3 N from its tip at right angles: the closing arrow measures 5 squares = 5 N.
Higher tier only. Free-body diagrams, resultants of forces at angles, scale drawings and resolving into right-angle components are Higher content in Eduqas 3.1(c).
3.1 · Forces and elasticity
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:
F = k xforce (N) = spring constant (N/m) × extension (m)
A straight line through the origin means F ∝ x; the gradient is the spring constant k. Past the limit it curves — Hooke's law no longer holds.
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. Specified practical SP3.1: hang masses on a spring and plot the force–extension graph.
Calculate
Your turn — Hooke's law
2A spring has a spring constant of 250 N/m. It is stretched by an extension of 0.12 m, staying below its limit of proportionality. Calculate the force applied.
N
Hint: F = k x = 250 × 0.12.
Mission 2 of 3
Why does a steel ship float?
Make a guess: why does the steel ship float but the nail sink?
3.2 · Pressure in fluids
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:
p = F / Apressure (Pa) = force normal to surface (N) ÷ area (m²)
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.
Worked example
A force of 200 N presses on an area of 0.5 m².
p = F ÷ A = 200 ÷ 0.5 = 400 Pa
Calculate
Your turn — pressure
3A box pushes down on the floor with a force of 600 N spread over an area of 0.4 m². Calculate the pressure on the floor.
Pa
Hint: p = F ÷ A = 600 ÷ 0.4.
3.2 · Higher tier only
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:
p = h ρ gpressure (Pa) = depth (m) × density (kg/m³) × gravitational field strength (N/kg)
The arrows show pressure pushing outwards; they grow longer with depth because there is more liquid above.
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.
Calculate · Higher
Your turn — pressure with depth
4Find the pressure due to the water 3 m below the surface of a lake. Water density ρ = 1000 kg/m³ and g = 9.8 N/kg.
Pa
Hint: p = h ρ g = 3 × 1000 × 9.8.
3.2 · Upthrust & floating · Higher tier
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:
It floats when upthrust equals weight. If weight is bigger than the most upthrust it can get, it sinks.
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.
Quick check · Higher
Float or sink?
?An object rests motionless, partly submerged and floating on water. What must be true about the upthrust and the object's weight?
Mission 2 check
Why does a steel ship float?
?A sheet of steel is pressed into a bowl, and it floats on water. The same sheet squashed into a solid lump sinks. Why does the bowl float?
Mission 3 of 3
Why does a longer spanner work better?
Make a guess: why does the longer spanner turn the nut?
3.3 · Moments, levers and gears
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:
M = F dmoment (Nm) = force (N) × distance normal to the force (m)
The force is at 90° to the handle, so the full length d counts. A longer spanner (bigger d) gives a bigger moment for the same 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.
Calculate
Your turn — moment
5A force of 20 N is applied at right angles to a spanner, 0.25 m from the pivot. Calculate the moment.
Nm
Hint: M = F d = 20 × 0.25.
3.3 · Balancing
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.
Left: 30 N × 2 m = 60 Nm. Right: 20 N × 3 m = 60 Nm. Equal moments → it balances.
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.
Calculate
Your turn — balancing the seesaw
6A child of weight 300 N sits 1.2 m from the pivot of a seesaw. How far from the pivot must a 450 N child sit on the other side to balance it?
m
Hint: balance means 300 × 1.2 = 450 × d, so d = 360 ÷ 450.
Mission 3 check
Why does a longer spanner work better?
?A stuck nut needs a moment of 60 N m to turn. Jo pushes at right angles with 200 N on a 0.25 m spanner, but it won't move. With a 0.40 m spanner and the same push, it turns. Why?
Sort it
Contact or non-contact?
Tap a force, then tap the box it belongs in.
🤝 Contact
🌍 Non-contact
Match it
Pick the right equation
Tap the equation you'd use for each situation.
Recap
The equations to know
Weight: W = m g
Hooke's law: F = k x
Pressure: p = F ÷ A
Pressure in a liquid (Higher): p = h ρ g
Moment: M = F d (d perpendicular to force)
Balanced: total clockwise moment = total anticlockwise moment
You've covered the key ideas across the three parts of Eduqas Topic 3 — forces & their interactions, pressure in fluids, and moments, levers & gears. Press Finish to see your score.
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Mini-lesson complete!
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You've worked through Forces for Eduqas GCSE Physics. 🎉