P5.2 Work Done and Energy Transfer
Work done
When a force causes an object to move in the direction of the force, work is done. Work done is a transfer of energy.
Formula:
W = Fs
W = work done (J), F = force (N), s = distance moved in direction of force (m)
1 joule = 1 newton-metre (1 J = 1 N·m).
Important: If the force and movement are perpendicular, no work is done by that force (e.g. carrying a bag horizontally — normal force does no work; gravity does no work).
✦Worked example
A person pushes a shopping trolley with a force of 25 N over a distance of 8 m.
W = Fs = 25 × 8 = 200 J
200 J of energy is transferred from the person's store (chemical energy) to kinetic energy of the trolley and thermal energy (due to friction).
Work done against friction — thermal energy
When friction acts on a moving object, work is done against the friction force. This energy is transferred to thermal energy (heat) in the surfaces and surroundings.
Example: A car decelerates due to brakes. Kinetic energy → thermal energy in brakes and surroundings. This is why brakes get hot.
Example: A meteor entering Earth's atmosphere — air resistance does work against the meteor; energy transferred to thermal energy → meteor glows and may burn up.
Mechanical work and energy stores
Work done IS energy transferred:
| Situation | Input energy store | Output energy store |
|---|---|---|
| Lifting an object | Chemical (muscles) | Gravitational PE |
| Stretching a spring | Mechanical / chemical | Elastic PE |
| Pushing against friction | Chemical / kinetic | Thermal |
| Driving a car | Chemical (fuel) | KE + thermal |
Power
Power is the rate of doing work (energy transfer per second):
P = W/t
P = power (W), W = work done (J), t = time (s)
1 watt = 1 joule per second.
Rearranged: W = Pt, t = W/P.
✦Worked example— Worked example — power
A motor lifts a 20 kg box 3 m in 5 seconds. g = 10 N/kg.
W = Fxs = mgs = 20 × 10 × 3 = 600 J
P = W/t = 600/5 = 120 W
Common exam errors
- Using the wrong distance — must be distance in direction of the force, not total path length.
- Confusing work and power — work is total energy transferred; power is the rate.
- Forgetting units — work in J, force in N, distance in m.
- Saying "no work done" when a force acts but there is no displacement.
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