This mini-lesson covers the whole of Edexcel Topic 15 — Forces and matter: how forces change the shape of objects (Hooke's law and the energy stored in a spring), and how pressure works in fluids — with depth, density, upthrust and the atmosphere.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Some screens are marked HT only (higher tier) or separate science. Press Start when you're ready.
To stretch, bend or compress an object you need more than one force acting on it. A single force would just make it accelerate away — to deform it, the object must be held or squeezed from at least two sides.
Edexcel splits the result of that deformation into two types:
Analogy: a trampoline springs back (elastic); chewing gum stretched and let go stays stretched (inelastic).
For a spring stretched elastically, the force is directly proportional to the extension:
The spring constant k measures stiffness: a stiffer spring needs a bigger force for the same extension. Rearrange to find it: k = F ÷ x.
Watch out: x is the extension (how much longer the spring becomes), not its total length. Always subtract the original (natural) length first.
A force of 12 N stretches a spring by 0.04 m.
k = F ÷ x = 12 ÷ 0.04 = 300 N/m
Plot force against extension and at first you get a straight line through the origin — a linear relationship where Hooke's law holds. Beyond the limit of proportionality the line bends: the relationship becomes non-linear and Hooke's law no longer applies.
Watch out: Hooke's law only holds up to the limit of proportionality. Past it, equal extra forces give bigger and bigger extensions, so F = k×x can no longer be used.
Stretching a spring does work, which is transferred to its elastic potential store. The energy stored is the area under the force–extension line (a triangle) — which gives:
Because the area under the straight line is a triangle, its area = ½ × base × height = ½ × x × F, and since F = k×x this rearranges to ½ k x².
Watch out: the extension is squared, so doubling the extension stores four times the energy. Use this equation only within the limit of proportionality.
A spring (k = 200 N/m) is stretched by 0.10 m.
E = ½ × 200 × 0.10² = ½ × 200 × 0.01 = 1.0 J
Tap a situation, then tap the box it belongs in.
A fluid (a liquid or a gas) presses on every surface it touches. The force always acts normal (at right angles) to the surface. Pressure is that force spread over an area:
One pascal (Pa) is one newton per square metre. The smaller the area, the bigger the pressure for the same force — which is why a sharp knife or a drawing pin works.
Separate science: this and the rest of Topic 15 from here on (15.7–15.17) are Physics-only (not Combined Science).
A force of 40 N acts on an area of 0.02 m².
p = F ÷ A = 40 ÷ 0.02 = 2000 Pa
Pressure in a fluid is partly atmospheric pressure (the air pushing down on top) and partly the weight of the fluid above. So the pressure in a liquid increases with depth — and is greater in a denser liquid, because a deeper or denser column weighs more.
Watch out: liquid pressure depends only on depth and density (and g) — not on the shape of the container or the area of its base. At the same depth, the pressure is the same in a thin tube and a wide tank.
The pressure from the liquid itself (above any atmospheric pressure) can be calculated:
To find the difference in pressure between two depths, use the difference in height (Δh) in the equation. Use g = 9.8 N/kg (some questions use 10).
Depth 2.0 m in water (ρ = 1000 kg/m³, g = 10 N/kg).
p = h × ρ × g = 2.0 × 1000 × 10 = 20 000 Pa
HT only. This equation and the explanation of why liquid pressure varies with depth and density are Higher Tier.
Because pressure is greater at the bottom of a submerged object than at the top, the fluid pushes up more than it pushes down. This net upward force is the upthrust. It equals the weight of the fluid displaced.
Float or sink? Compare the density of the object with the density of the fluid: less dense → floats; more dense → sinks. A floating object displaces its own weight of fluid. HT only.
Tap the correct outcome for each object placed in water (density 1000 kg/m³).
The atmosphere is a "sea" of air. Its pressure comes from the weight of the air above you. As you go higher, there is less air above pressing down, and the air is less dense — so atmospheric pressure decreases with height.
Deformation: elastic (springs back) vs inelastic/plastic (stays changed); deforming needs >1 force.
Hooke's law: F = k × x (only up to the limit of proportionality).
Energy stored: E = ½ × k × x² (area under the force–extension line).
Pressure: p = F ÷ A — force normal to a surface ÷ area. (separate science)
Liquid column (HT): p = h × ρ × g — pressure rises with depth & density.
Upthrust (HT): = weight of fluid displaced; float if less dense than the fluid.
Atmosphere: pressure falls with height (less air above).
You've covered all of Edexcel Topic 15 — Forces and matter, with the HT and separate-science parts flagged. Press Finish to see your score.
You've worked through Forces and matter for Edexcel GCSE Physics. 🎉
Your stars: 0 / 0
Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.