A simple machine is a device with no moving parts, or one moving part, that makes a force easier to use. There are six, taught in Grade 3 to Grade 8 science and again in physics when work and energy arrive, and the whole topic rests on one idea: a machine can trade a smaller force for a longer distance, but it cannot do less work than the job needs. The six, with examples, mechanical advantage and the classes of lever, are below with the printable chart.
Download the simple machines chart (PDF) (the six machines with examples and mechanical advantage on page 1, the three classes of lever, compound machines and a practice set with answers on page 2; two pages, free).
The six simple machines
| Machine | How it helps | Everyday examples | Mechanical advantage |
|---|---|---|---|
| Lever | a rigid bar turning about a fulcrum; a small effort far from the fulcrum moves a big load close to it | seesaw, crowbar, scissors, wheelbarrow, tweezers, a spoon flicking a pea | effort arm ÷ load arm |
| Wheel and axle | a big wheel turning a small axle multiplies force (or a turned axle multiplies the wheel's speed) | doorknob, steering wheel, screwdriver handle, bicycle pedals, a tap | wheel radius ÷ axle radius |
| Pulley | a wheel with a rope in its groove; changes the direction of the pull, and with several wheels shares the load between rope sections | flagpole, blinds, crane, well, gym cable machine | number of rope sections holding the load |
| Inclined plane | a sloped surface; lifting along the slope needs less force than lifting straight up | ramp, slide, staircase, a road up a hill | slope length ÷ height |
| Wedge | two inclined planes back to back, pushed into something to split or lift it | knife, axe, nail, doorstop, chisel, your front teeth | wedge length ÷ thickness of the thick end |
| Screw | an inclined plane wrapped round a cylinder; a turn moves it a short way with a big force | screw, bolt, jar lid, drill bit, spiral staircase | circumference of the turn ÷ distance advanced per turn (the pitch) |
The first three (lever, wheel and axle, pulley) are all turning machines; the last three (inclined plane, wedge, screw) are all slopes. That grouping is how most textbooks organise them and is worth remembering as a pair of threes.
Why a machine cannot cheat: work in = work out
Work is force × distance. A ramp 4 m long and 1 m high lets you push a 400 N box up with about 100 N instead of lifting it with 400 N, but you push it 4 m instead of 1 m: 100 × 4 = 400 × 1. The work is the same (and in practice a little more, because friction on the ramp wastes some). Every simple machine is this trade: less force, more distance, or the other way round. Nothing in physics lets a machine give out more work than you put in.
Mechanical advantage (MA) says how many times the force is multiplied: MA = load ÷ effort. The ideal MA, ignoring friction, equals the distance the effort moves ÷ the distance the load moves, which is why each machine's column in the table is a ratio of lengths. The actual MA is smaller, and efficiency = actual MA ÷ ideal MA (or work out ÷ work in), always under 100 percent.
The three classes of lever
| Class | In the middle | Examples | What it does |
|---|---|---|---|
| First | the fulcrum | seesaw, crowbar, scissors, pliers, a claw hammer pulling a nail | multiplies force or distance depending on where the fulcrum sits; reverses direction |
| Second | the load | wheelbarrow, nutcracker, bottle opener, a door on its hinges | always multiplies force (MA > 1); the effort moves farther than the load |
| Third | the effort | tweezers, fishing rod, broom, baseball bat, your forearm lifting a cup | multiplies distance and speed; the effort is bigger than the load (MA < 1) |
The memory aid: F-L-E, first-second-third, for what sits in the middle: Fulcrum, Load, Effort.
Compound machines
Two or more simple machines working together: scissors (two first-class levers plus two wedges at the blades), a wheelbarrow (a second-class lever on a wheel and axle), a bicycle (wheels and axles, levers, a gear train), a can opener (lever, wheel and axle, wedge), a crane (pulleys and levers). The question "which simple machines are in a ...?" is answered by naming each part's job.
Worked examples
- A lever lifts a 600 N rock with a 150 N push. MA = 600 ÷ 150 = 4.
- A ramp is 6 m long and 1.5 m high. Ideal MA = 6 ÷ 1.5 = 4; a 200 N load needs an ideal effort of 50 N.
- A pulley system has 3 rope sections supporting the load. Ideal MA = 3; a 90 N load needs 30 N, and the rope must be pulled 3 m to lift it 1 m.
- A screwdriver handle has radius 1.5 cm, the shaft 0.3 cm. MA = 1.5 ÷ 0.3 = 5.
- A machine takes 500 J of work in and gives 400 J out. Efficiency = 80 percent; the rest is heat from friction.
- Which class of lever is a wheelbarrow? The load (the barrow's contents) sits between the fulcrum (the wheel) and the effort (the handles): second class.
The mistakes that lose marks
- Saying a machine "reduces the work": it reduces the force, and the distance grows to match.
- Mixing up the lever classes: look at what is in the middle, and remember F-L-E.
- Counting the rope you pull as a supporting section in a pulley system when it pulls downward from the top block: only the sections that hold the moving block count.
- Treating a wedge as a different idea from an inclined plane: a wedge is a moving inclined plane, a screw is a wound one.
- Calling a bicycle or scissors a simple machine: they are compound machines.
Where to get help
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Related pages
- Net force, balanced and unbalanced forces
- Potential energy: gravitational and elastic
- The kinetic friction formula and the coefficient table
Questions parents ask
1What are the six simple machines?
2What does a simple machine do?
3What is mechanical advantage?
4What are the three classes of lever?
5Is a bicycle a simple machine?
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