The Six Simple Machines
The six simple machines are the basic mechanical devices identified by the ancient Greeks over 2,000 years ago. They are the building blocks of almost every more complex machine ever invented. Each one helps people do work (moving an object by a force) more easily, usually by trading a small force over a long distance for a big force over a short distance. The six are: lever, wheel and axle, pulley, inclined plane, wedge and screw. Look closely at any mechanical device and you will find one or more of them inside.
- LeverBar pivoted at a pointSeesaws, crowbars, scissors
- Wheel and axleWheel attached to a shaftCars, doorknobs, fans
- PulleyRope over a wheelCranes, flagpoles, blinds
- Inclined planeA slope or rampStairs, ramps, hills
- WedgeTwo slopes back-to-backKnives, axes, doorstops
- ScrewSlope wrapped around a cylinderWood screws, jar lids, drills
What is "work" in physics?
In physics, work means moving an object by applying a force over a distance. The amount of work equals the force multiplied by the distance: W = F x d. Lifting a 10 kg sack 2 metres takes the same total work whether you carry it straight up or push it up a long ramp.
What changes is the effort you need at any moment. A simple machine lets you use less force at the cost of moving over a longer distance. The total work done is the same.
1. Lever
A lever is a rigid bar that pivots at a fixed point called the fulcrum. Push down on one end and the other end rises. The closer the load is to the fulcrum, the easier it is to lift. Examples: seesaws, crowbars, wheelbarrows, scissors, bottle openers, fishing rods.
Archimedes once boasted: "Give me a place to stand and I will move the Earth", meaning a long enough lever could move anything.
2. Wheel and axle
A wheel and axle is a large wheel attached to a smaller shaft (the axle). Turning the wheel turns the axle (or vice versa), trading force for distance. Examples: car wheels, doorknobs, fans, screwdrivers, water wheels, ferris wheels.
3. Pulley
A pulley is a wheel with a rope, chain or belt running over it. A single fixed pulley changes the direction of a pull (pulling down to lift up). Multiple pulleys combined (a "block and tackle") can multiply your strength enormously. Examples: cranes, flagpoles, sailing rigs, well buckets, lifts, window blinds.
4. Inclined plane
An inclined plane is a flat slope. Pushing a load up a slope takes less force than lifting it straight up, but you have to travel a longer distance. The gentler the slope, the easier the push, but the further you have to go. Examples: ramps for wheelchairs, lorry loading ramps, hills, stairs, the angled cuts of a knife.
5. Wedge
A wedge is essentially two inclined planes back to back. When pushed into something, it splits it apart with great force. Examples: knives, axes, chisels, doorstops, the teeth of saws, the pointed front of a ship.
A wedge is an active simple machine: you push it forward, and it pushes things outward.
6. Screw
A screw is an inclined plane wrapped around a cylinder. Turning the screw moves it inward or outward, applying force from a turning motion. Examples: wood screws, jar lids, bolts, drill bits, light bulb fittings, hand-cranked car jacks, Archimedes screw for lifting water.
Combining simple machines
Most real-life devices combine several simple machines:
- Scissors: two levers joined at a fulcrum, with two wedge-shaped blades.
- Bicycle: wheels and axles, levers (pedals and brakes), screws, plus the wedges of the brake pads.
- Lawn mower: wheel and axle, wedges (the blades), levers (handles), screws holding it together.
- Manual can opener: a lever to apply force, a wedge to pierce the lid, a wheel and axle to drive the cutting wheel.
- Stairlift: screws, wheels, an inclined plane (the staircase).
- Crane: pulleys, levers, wheels, screws and many other elements.
Engineers call combinations of simple machines compound machines. Even the most complex machines (cars, helicopters, computers) trace back to these six basic ideas at their hearts.
Mechanical advantage
The amount a simple machine multiplies your force is called its mechanical advantage. A mechanical advantage of 4 means the machine lets you lift a weight 4 times heavier than you could lift directly. The trade-off is that you have to apply the force over 4 times the distance.
Deeper dive: how the ancient pyramids might have been built
The Great Pyramid of Giza, finished about 4,500 years ago, is one of the most impressive constructions in history. It consists of around 2.3 million stone blocks, each weighing between 2.5 and 80 tonnes. How the ancient Egyptians moved these massive stones into place is still debated by archaeologists.
What we do know is that they must have used simple machines:
- Inclined planes: most experts agree that long ramps of earth and rubble were built spiralling up the pyramid as it grew. Stones were dragged up the ramp on wooden sleds, with people pouring water on the sand to reduce friction (a recent discovery in an Egyptian tomb painting). The ramps were dismantled when the pyramid was finished.
- Levers: large stones could be tilted and turned with wooden levers, used to lift one end of a block onto a roller or to wedge it into place.
- Rollers: blocks were placed on wooden rollers and rolled along the ground, a kind of primitive wheel.
- Pulleys: archaeologists are not sure whether the Egyptians had true pulleys, but they certainly used ropes over fixed points to change the direction of pull.
- Wedges: stones were cut from the quarry using copper chisels (wedges) hammered into cracks, then split with wooden wedges soaked with water to expand.
Modern engineers have tried to replicate the techniques and confirm that the work could have been done by tens of thousands of skilled workers over decades, with no need for any mystery technology. The simple machines we know today, plus a lot of organisation and human effort, were enough to build one of the wonders of the ancient world.