Lesson focus: Mechanical work transfers energy when a forceA push or a pull, measured in newtons (N). moves an object, while machineA device that makes a job easier by changing the size or direction of a force, like a lever. trade force against distance rather than creating energy.
Calculate work and explain ideal and real machine trade-offs.
Build on earlier learning
Turning forcesTurning forces and levers introduced force multiplication; this lesson accounts for the associated energy transfer.
Practical
Work done with a pulley
- Lift a load directly and measure the force with a newtonmeter.
- Lift it with a pulley system and measure the force and the distance pulled.
- Work done = force × distance; compare the two.
Record: A table of force, distance and work done for each method.
You will be successful when you can…
- Define mechanical work.
- Use W = Fd in aligned cases.
- Explain force-distance trade-offs.
- Account for machine efficiency.
Words that carry the science
- work done
- Energy transferred when a force causes displacement.
- joule
- The unit of energy and work.
- force
- A push or pull measured in newtons.
- mechanical advantage
- Output force divided by input force.
- machine
- A device that changes force, distance, direction or speed to help perform a task.
Work is an energy transfer
01Work is an energy transfer
When a forceA push or a pull, measured in newtons (N). moves an object in the force direction, work doneThe energy transferred when a force moves an object: work done = force × distance moved in the direction of the force. = force × distance moved in that direction.
One jouleThe unit for measuring energy (J). of work occurs when a force of one newton moves its point of application one metre in the force direction.
02Machines trade force for distance
A ramp, lever or pulley is a machineA device that makes a job easier by changing the size or direction of a force, like a lever. that can reduce the input force by increasing the distance over which that force acts, or change force direction.
For an ideal machineA device that makes a job easier by changing the size or direction of a force, like a lever., input work equals useful output work. A smaller force therefore requires a proportionally larger input distance.
03Real machines dissipate energy
Friction and deformation transfer some input energy into thermal stores and sound, so useful output work is less than input work.
mechanical advantageHow many times a machine multiplies the force you put in. alone does not give efficiency. Measure both force and distance on input and output sides.
Simple machines: levers and pulleys
Simple machines make a job easier by reducing the force needed. The catch: you must move the effort further. For an ideal machine, work done = force × distance is the same on both sides, so energy is conserved.
Controls
This is an ideal machine with no friction and a weightless lever and pulleys. Real machines need a little more effort.
Analyse an ideal ramp
A 400 N load rises 0.50 m using a 2.0 m ramp.
- Useful output work = 400 × 0.50 = 200 J.
- For an ideal ramp, input work is also 200 J.
- Input force = work ÷ distance = 200 ÷ 2.0 = 100 N.
- The fourfold distance gives a fourfold force reduction in the ideal case.
How to make the reasoning visible: The ramp changes the force-distance combination without reducing ideal work.
Work and machines
Compare measured machine inputs with useful outputs.
| Machine | Input work / J | Useful output / J | Efficiency |
|---|---|---|---|
| ramp A | 240 | 200 | 83% |
| pulley B | 180 | 150 | 83% |
| lever C | 125 | 100 | 80% |
| ramp D | 330 | 300 | 91% |
Which machine dissipates the most energy?
Why can pulley B reduce force without reducing ideal work?
Check your understanding
Answer from memory, then check against the suggested answer.
Work equation for aligned force?
Unit of work?
Can a machine create energy?
A 30 N force pushes a box 4 m in the same direction. Calculate the work done.
Why can a machine have high mechanical advantage but low efficiency?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A ramp lets a worker raise a crate with a smaller force.
Explain why the ramp does not save energy in an ideal model.
(4)Explain why a real ramp needs more work.
(2)Total for this question: 6 marks
Command word: Link the smaller force to greater distance, then add the real effect of friction.