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IB DemystifiedMYP Sciences

Work, power and efficiency

Lifting a bag, pushing a trolley or pumping water from a well all involve doing work: transferring energy with a force. Power tells us how quickly that work is done, and efficiency tells us how much of the energy we put in is actually useful.

Recommended for MYP 3 · About 3 lessons · Criteria A, B, C and D

3.0 mAli: weight 600 Ntime to the top: 4.0 sSana: weight 500 Ntime to the top: 3.0 sOnly the vertical height matters when calculating work done against gravity.
Figure 1. The same climb can involve different amounts of work and power.
On this page
  1. Learning objectives
  2. Before you start
  3. Key vocabulary
  4. Understanding the ideas
  5. Key equations
  6. Work and power in the real world
  7. Worked examples
  8. Assessment tips
  9. Check your understanding
  10. Practice questions
  11. Investigation
  12. Criterion-linked questions
  13. Challenge questions
  14. Topic check
  15. Review your mistakes
  16. Your progress

Learning objectives

By the end of this topic you should be able to:

  • calculate work done using W = F × d
  • calculate power using P = W ÷ t and energy using E = P × t
  • calculate and interpret efficiency
  • explain how ramps, pulleys and levers change force and distance
  • analyse work and efficiency data and design investigations
  • discuss transport choices and efficient appliances

Before you start

You will use these skills. If any feel shaky, review them first.

  • energy stores and efficiency (see Energy stores and transfers)
  • forces and weight (see Newton's laws of motion)
  • moments and levers (see Moments and levers)

Key vocabulary

Work done
Energy transferred when a force moves an object: W = F × d, in joules (J).
Power
The rate of doing work or transferring energy: P = W ÷ t, in watts (W).
Watt
One joule per second.
Efficiency
Useful energy out ÷ total energy in × 100%.
Wasted energy
Energy transferred to stores that are not useful, often heat from friction.
Machine
A device that changes the size or direction of a force, such as a ramp, lever or pulley.

Understanding the ideas

  1. Work

    Work is done when a force moves an object in the direction of the force: work done = force × distance. Lifting something means doing work against gravity: weight × vertical height. If nothing moves, no work is done, however tiring it feels.

  2. Power

    Power is how quickly work is done: power = work ÷ time, measured in watts. Two people can do the same work, but the faster one has more power. Energy transferred by an appliance = power × time.

  3. Efficiency

    No machine is 100% efficient, because some energy is always wasted, usually by friction heating moving parts. Efficiency = useful energy out ÷ total energy in × 100%. Lubricating parts and using better designs improve efficiency.

  4. Machines

    Machines such as ramps, pulleys and levers let us use a smaller force over a longer distance. They do not reduce the work needed; in fact, friction means they need slightly more.

  5. What does it connect to?

    This topic links energy, forces and moments, and leads to electrical power, energy resources and engines.

Key equations

  • Work done (J) = force (N) × distance (m).
  • Work against gravity = weight (N) × vertical height (m).
  • Power (W) = work done (J) ÷ time (s).
  • Energy (J) = power (W) × time (s).
  • Efficiency (%) = useful energy (or power) out ÷ total energy (or power) in × 100.

Work and power in the real world

Cranes on building sites are rated by the power of their motors. Electric rickshaws and motorbikes are replacing pedal power on some city streets. Efficient fans and motors help families cut electricity bills in hot summers.

Worked examples

Example 1: work and power

A 700 N student climbs 4.0 m of stairs in 5.0 s. Find the work done and the power.

  1. Work = 700 × 4.0 = 2800 J.
  2. Power = 2800 ÷ 5.0 = 560 W.

Example 2: efficiency

A motor is supplied with 400 J and lifts a load, doing 300 J of useful work. Find its efficiency.

  1. Efficiency = 300 ÷ 400 × 100.
  2. = 75%. The other 100 J is wasted, mostly as heat.

Assessment tips

Questions on work, power and efficiency often combine two or more equations. Expect to:

  • Calculate work, power, energy and efficiency with correct units.
  • Rearrange equations to find force, distance or time.
  • Explain how machines trade force for distance.
  • Analyse data and evaluate claims about machines.

Common mistakes: using the slope distance instead of the vertical height when lifting; forgetting to convert minutes to seconds; confusing work with power; and thinking machines reduce the work or energy needed.

Check your understanding

Quick questions on the ideas above. Try each one before using a hint.

Practice questions

Show

Investigation: how efficient is a pulley?

Partially guided investigation · about 40 minutes · pairs

Research question
How does the number of pulleys in a system affect its efficiency?
Scientific background
A pulley system reduces the effort force, but the effort must move further. Friction in the pulleys wastes some energy, so efficiency is less than 100%.
Hypothesis
Write your own prediction, with a scientific justification.
Variables
Identify your independent, dependent and control variables, and explain how you will control them.
Apparatus
Pulley blocks (single and double), string, clamp stand, a 10 N load, newton meter, metre rule.
Method
  1. Set up a single pulley and lift the 10 N load 0.20 m at a steady speed, reading the effort force on the newton meter.
  2. Measure how far the effort end of the string moves.
  3. Calculate useful work (10 N × 0.20 m) and work put in (effort × effort distance), then the efficiency.
  4. Repeat with two and four pulleys, three times each, and find the mean efficiency.

Safety. Clamp the stand firmly so it cannot topple, keep feet clear of the load, and lift slowly.

Then evaluate: why does the efficiency change as more pulleys are added?

Criterion-linked questions

Criterion B: inquiring and designing

Criterion C: processing and evaluating

Criterion D: reflecting on the impacts of science

Challenge questions

Harder problems in unfamiliar contexts. Plan before you calculate.

Topic check

Five questions picked at random from the whole topic. Take a new set whenever you like.

Review your mistakes

Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.

Your progress

Tracked separately for each skill, on this device only.

SkillCorrectStatus

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