Energy is the currency of physics: never created or destroyed, only moved between stores and changed in form. Work is the transfer of energy by a force, and power is how fast that transfer happens.
Work done = force × distance moved in the direction of the force, W = Fs cosθ. Energy is conserved: the total stays constant as it transfers between stores. Power is the rate of doing work, P = W / t, and for a constant force moving at speed v, P = Fv. Efficiency is the useful output divided by the total input.
A constant force drags a crate at a steady speed against friction. Watch the work done build up and the power read out as P = Fv. Change the force and the speed and see how the power responds.
Match each to the situation: work for an energy transfer, power for a rate, efficiency for a ratio.
| Relation | Meaning | Unit |
|---|---|---|
| W = Fs cosθ | energy transferred by a force | J |
| P = W / t = Fv | rate of transferring energy | W |
| efficiency = useful / total | fraction usefully transferred | no unit (or %) |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
Work done by a force is defined as the force multiplied by the:
A waiter carries a tray horizontally at constant height. The work done by the upward force supporting the tray is:
A motor does 6000 J of work in 30 s. Its power output is:
A car engine provides a constant driving force of 500 N while the car moves at 20 m s⁻¹. The power developed is:
Conservation of energy lets you track every joule from input to useful output and waste.
Two slips recur. Work is zero when the force is perpendicular to the motion, so carrying a bag horizontally does no work against gravity. And when a force acts at an angle, you must include cosθ, using only the component along the motion. Efficiency can never exceed 100%; if your answer does, you have swapped useful and total.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A 60 N force pulls a sledge 8.0 m horizontally while acting at 30° above the horizontal. The work done by the force is:
A crane lifts a 200 kg load at a steady 0.50 m s⁻¹. Taking g = 9.8 m s⁻², the useful output power is:
A lamp is supplied with 60 W of power and emits 9.0 W as light. Its efficiency is:
This skill is now lit gold on your star map. Keep the chain going.