Work, energy and power
Work done
When a force moves an object in the direction of the force, work is done:
W = F × d
- W = work in joules (J)
- F = force in newtons (N)
- d = distance moved in the direction of the force, in metres (m)
One joule is the work done when a force of 1 N moves an object 1 m. Work done equals the energy transferred — they are the same physical quantity.
Where does the energy go?
When you do work against a force, energy is transferred:
- Work against gravity (lifting a box) → gravitational potential energy.
- Work to accelerate (push a trolley) → kinetic energy.
- Work against friction → thermal energy in surfaces (objects warm up — handy in fire-by-friction demos).
Power
Power is the rate of doing work or the rate of energy transfer:
P = E / t = W / t
- P = power in watts (W). 1 W = 1 J/s.
- E = energy transferred in joules (J).
- t = time in seconds (s).
A 60 W lamp transfers 60 J every second. A typical kettle is ~2.5 kW = 2500 W.
✦Worked example— Worked examples
A weightlifter lifts a 80 kg barbell 2 m vertically in 1.5 s (g = 10 N/kg):
- Force needed = weight = mg = 800 N.
- Work done = F × d = 800 × 2 = 1600 J.
- Power = 1600 / 1.5 ≈ 1067 W.
A 40 W LED bulb runs for 5 hours:
- t = 5 × 3600 = 18 000 s.
- E = P × t = 40 × 18 000 = 720 000 J = 720 kJ.
Edexcel exam tip
Always check the units before substituting. Common pitfalls: time given in minutes, force given in kg (need to multiply by g first), distance in cm. Watch for these in two-mark calculation questions.
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science-leaves