Simple Machines
Six devices that trade distance for force.
A simple machine changes the direction or magnitude of a force. There are six, and every complex mechanism is a combination of them. None creates energy — they trade force against distance.
| Machine | What it does | Examples |
|---|---|---|
| Lever | Pivots about a fulcrum | Crowbar, seesaw, scissors |
| Wheel and axle | Rotates a large wheel to turn a small axle | Doorknob, steering wheel |
| Pulley | Redirects force; multiple pulleys multiply it | Flagpole, block and tackle |
| Inclined plane | Raises a load over a longer distance | Ramp, road up a hill |
| Wedge | A moving inclined plane that splits | Axe, knife, chisel |
| Screw | An inclined plane wrapped round a cylinder | Bolt, jar lid, drill |
Mechanical advantage
An advantage of 4 means a quarter of the force is needed — and four times the distance must be moved. That trade is not a limitation of the design; it follows from conservation of energy.
Work stays the same
Work is force times distance. A machine cannot reduce the work needed, only redistribute it. In practice output is always less than input because friction converts some to heat, which is why no machine reaches 100% efficiency.
Lever classes
First class: fulcrum between effort and load — a seesaw. Second class: load between fulcrum and effort — a wheelbarrow, always giving advantage. Third class: effort between fulcrum and load — tweezers and the human forearm, which sacrifice force for speed and range of movement.