GCSE Physics Work and power revision

Work done is the energy transferred when a force moves an object along the direction of the force: W = F s, force in newtons, distance in metres, work in joules. The distance must be along the line of action of the force — carrying a heavy bag levelly along a corridor does no work against gravity, because the bag’s motion is at right angles to the weight. When you lift a mass, the force you apply is the weight (m g), so lifting 80 kg by 2.0 m takes W = 80 × 9.8 × 2.0 = 1 568 J ≈ 1 600 J. Friction means the person usually transfers more energy than the useful output: the difference warms the surfaces — energy dissipated to the thermal store of the surroundings.

Power is the rate of energy transfer: P = W/t, work in joules, time in seconds, power in watts. One watt means one joule transferred each second. Machines are rated by the power at which they transfer energy: the same 1 000 J of work done in half the time needs twice the power. When something lifts a load, the work done per second is weight × height ÷ time — a 70 kg climber rising 8.0 m in 20 s transfers about 5 490 J, a power of roughly 270 W, comparable to a bright old-style filament lamp.

Stretching a spring, the pull and the stretch grow together: F = k e, where e is the extension (the extra length beyond the natural length) and k is the spring constant in newtons per metre (N/m). A stiff spring has a large k. This proportionality holds only up to the limit of proportionality; past it the graph curves, and if stretched further still the spring deforms inelastically — it keeps some permanent stretch. A material that returns to its original shape when the force is removed was elastically deformed; one that doesn’t was inelastically deformed.

1. 1Set up

   Hang a spring from a clamp stand and fix a mm ruler alongside, reading the natural length with the pointer at the spring’s end (or the bottom coil).

2. 2Load and read

   Hang a known weight (for example 1.0 N steps from a slotted mass set) and record the new length; the extension = new length − natural length.

3. 3Repeat and unload

   Add weights one at a time up to six or so readings, then unload them to check the spring returns to its natural length (elastic behaviour).

4. 4Plot and interpret

   Plot force (N) on the y-axis against extension (m) on the x-axis. The straight portion through the origin shows Hooke’s law; its gradient is the spring constant k in N/m.

5. 5Control variables

   Same spring, vertical ruler aligned with the pointer, eye level with the reading to avoid parallax, masses added gently with no oscillation.

The work done stretching a spring is stored as elastic potential energy. In the straight-line (Hooke) region that stored energy is E e = ½ k e² (Higher) — which is also the area under the force–extension graph up to that extension. Because extension is squared, small increases in stretch store disproportionately more energy: doubling the extension quadruples the energy stored. Catapults, bows and spring-loaded toys all run on this store — releasing the spring transfers the elastic store to the kinetic store of the projectile.