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AS · Practice questions · Work, energy and power

Transfer it, time it, account for it.

Six original Cambridge-style questions on work done including a force at an angle, power as a rate and as P = Fv, and efficiency.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Force, distance, time.

01
Analysis
[2 marks]

State what is meant by the work done by a force, and give its SI unit.

  • Work done = force × distance moved in the direction of the force ✓
  • The SI unit is the joule (J) ✓
02
Analysis
[3 marks]

A 25 N force drags a box 6.0 m along the floor in the direction of the force, taking 3.0 s. Find the work done and the average power.

  • Work = Fs = 25 × 6.0 = 150 J ✓
  • Power = W / t = 150 / 3.0 = 50 W ✓
03
Analysis
[3 marks]

A force of 80 N is applied at 25° to the horizontal and moves an object 5.0 m horizontally. Find the work done by the force.

  • Use the component along the motion: W = Fs cosθ ✓
  • W = 80 × 5.0 × cos 25° = 80 × 5.0 × 0.906 ✓
  • W = 363 J ✓
04
Analysis
[3 marks]

A pump raises 300 kg of water through a height of 12 m every minute. Taking g = 9.8 m s⁻², find the useful output power.

  • Work each minute = mgh = 300 × 9.8 × 12 = 35 280 J ✓
  • Power = work / time = 35 280 / 60 ✓
  • Power = 588 W ✓
05
Analysis
[2 marks]

An electric motor is supplied with 500 W and delivers 350 W of mechanical power. Find its efficiency.

  • Efficiency = useful / total = 350 / 500 ✓
  • Efficiency = 0.70 = 70% ✓
06
Analysis
[3 marks]

A car travels at a steady 30 m s⁻¹ against a total resistive force of 600 N. Find the driving force and the power developed.

  • At constant speed the driving force balances the resistance, so it is 600 N ✓
  • Power = Fv = 600 × 30 = 18 000 W = 18 kW ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.