A simple machine makes a job feel easier by changing how a force is applied. What it never does is reduce the work. Pushing a load up a ramp takes a smaller force than lifting it straight up, but you push over a longer distance, and the two changes cancel out.
The six simple machines
| Machine | What it does | Everyday examples |
|---|---|---|
| Lever | Turns a small force over a long distance into a large force over a short one, pivoting on a fulcrum | Seesaw, crowbar, scissors, a broom held in two hands |
| Wheel and axle | A large wheel turns a small axle with more force, or the reverse | Doorknob, screwdriver handle, steering wheel |
| Pulley | Changes the direction of a pull, and with several wheels shares the load | Flagpole, window blind, crane |
| Inclined plane | Raises a load with a smaller force over a longer path | Ramp, staircase, loading dock |
| Wedge | Two inclined planes back to back, turning a push into a sideways split | Axe, knife, chisel, doorstop |
| Screw | An inclined plane wound around a cylinder | Wood screw, jar lid, bolt |
The trade that every machine makes
Work is force multiplied by distance. A machine can lower the force it takes to do a job, but then the distance grows in step. This is the point students most often miss, and it is easy to feel with a ramp.
Lifting a box one metre, two ways
straight up: large force x 1 metre up a 4 m ramp: quarter force x 4 metres the product, which is the work, is the same
Friction means the ramp actually takes slightly more work in the real world. No machine returns more than it is given.
Levers and the fulcrum
A lever has three parts: the effort you apply, the load you are moving, and the fulcrum it pivots on. Where the fulcrum sits decides what the lever is good for.
- Fulcrum in the middle. Seesaw, scissors, pliers. Effort and load sit on opposite sides.
- Load in the middle. A wheelbarrow. The fulcrum is the wheel, and you lift the handles.
- Effort in the middle. Tongs, tweezers, a fishing rod. These cost you force but buy speed and reach at the far end.
Moving the fulcrum closer to the load makes the effort smaller and the distance you move your hand larger. That is the same trade again.
Compound machines
Most tools are several simple machines working together. A pair of scissors is two levers sharing a fulcrum, each blade a wedge. A bicycle combines wheels and axles, levers in the brakes, and a screw in every bolt holding it together.
Check yourself
Does a ramp reduce the work needed to lift a box?
No. It reduces the force, and increases the distance by the same factor. With friction, a ramp needs slightly more total work than a straight lift.
Which simple machine is a doorknob, and why?
A wheel and axle. Your hand turns the wide knob through a large circle, which turns the narrow shaft with more force than your fingers could apply to the shaft directly.
Why is a knife blade a wedge and not just a flat tool?
Because its two sloped faces meet at an edge. Pushing down along that slope turns a small downward push into a much larger sideways force that separates the material.
Where should the fulcrum go to lift a heavy rock with a crowbar?
As close to the rock as you can get it. The shorter the load arm and the longer the effort arm, the less force your hands have to supply, though your hands then travel further.
See also
Sources
- The six simple machines and the force-distance trade are standard content in United States elementary physical science standards. Examples on this page are our own.