Defining power as the rate of doing work, calculating power and efficiency, and explaining why no machine is 100% efficient and how its efficiency can be improved.
4 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
A crane and a person can both lift a load to the same height. The crane does it faster — it has more power.
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Copy these into your notebook in your own words — that is how they stick.
Rate of doing work / transferring energy; P = W/t = E/t.
1 W = 1 J/s; 1 kW = 1 000 W.
E = Pt.
(useful energy output ÷ energy input) × 100%.
(useful power output ÷ power input) × 100%.
Energy is wasted as thermal energy and sound.
Lubricate, insulate, better design.
Useful output can never be larger than the input.
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness. More games in the Physics Arcade.
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better. Work each item on paper first.
Write full answers, then reveal the mark scheme and tick the points you earned (14 marks in total). Show all your working — method marks count. Your answers are saved on this device.
Question 1 [14 marks]
A small electric motor lifts a suitcase of weight 80 N through a height of 1.5 m in 5.0 s. The motor takes 200 J of electrical energy.
(a) (a) Define power. [1]
(b) (b) Calculate the useful work done in lifting the suitcase. [3]
(c) (c) Calculate the useful power output of the motor. [3]
(d) (d) Calculate the efficiency of the motor. [3]
(e) (e) State what happens to the 80 J of energy that is not useful. [2]
(f) (f) Suggest two ways to increase the efficiency of the motor. [2]
Find these in your own past-paper booklets for extra timed practice. Items above are written in the style of these papers; any adapted from a specific question say so.