A ramp, a lever, and a screw all do the same clever trick — they don't reduce the work needed, they just make it easier to apply.
Push a heavy box across the floor, and physics has a very specific opinion about whether you did any "work" at all — one that has almost nothing to do with how tired you felt. In physics, work has an exact meaning, and understanding it unlocks a genuinely clever family of tools: simple machines, which have let humans move loads far heavier than their own muscles allow, for thousands of years, without adding a single watt of extra power.
In physics, work is done only when a force causes an object to move in the direction of that force. Holding a heavy box perfectly still, no matter how much effort it takes, technically counts as zero work in physics terms — because nothing moved. Carry that same box across a room, however, and work has genuinely been done.
Work = Force × Distance (W = Fd)
Pushing an object with 20 newtons of force across 5 metres does 20 × 5 = 100 joules of work.
Work is measured in joules (J) — the same unit used for energy, which is not a coincidence. Work is really just a transfer of energy: when you do work on an object, you're giving it energy, usually as kinetic or potential energy.
Power measures something different: not how much work is done, but how quickly it's done.
Power = Work ÷ Time (P = W/t)
Doing 100 joules of work in 2 seconds gives 100 ÷ 2 = 50 watts of power.
Two people can do the exact same amount of work — say, carrying identical boxes up the same flight of stairs — but the person who does it faster is more powerful, even though neither one did more work. Power is the rate at which energy is transferred or converted.
| Quantity | Formula | Unit |
|---|---|---|
| Work | Force × Distance | Joule (J) |
| Power | Work ÷ Time | Watt (W) |
The watt is named after James Watt (1736–1819), a Scottish engineer whose improvements to the steam engine helped power the Industrial Revolution. Watt didn't invent the steam engine, but his redesigns made it dramatically more efficient and practical for widespread industrial use. In recognition of his contributions, the name "watt" was proposed for the unit of power in 1882 and formally adopted in 1889 — decades after his death — and it remains the standard unit of power today.
One watt equals exactly one joule of work done per second. A 100-watt light bulb uses 100 joules of energy every single second it's switched on.
A simple machine is a basic mechanical device that changes the direction or size of a force, making a task easier to perform — even though, as we'll see, it never actually reduces the total work required. There are traditionally six recognised types.
| Simple Machine | How It Works | Everyday Example |
|---|---|---|
| Lever | A rigid bar that pivots around a fixed point (fulcrum) | A seesaw, a crowbar |
| Wheel and Axle | A wheel fixed to a rod; turning one turns the other | A doorknob, a steering wheel |
| Pulley | A grooved wheel with a rope, changing the direction of a pulling force | A flagpole, a well bucket |
| Inclined Plane | A sloped surface that spreads a lift over a longer distance | A ramp, a mountain road |
| Wedge | Two inclined planes joined back to back, used to split things apart | An axe head, a doorstop |
| Screw | An inclined plane wrapped around a cylinder | A jar lid, a screw fastener |
Every complex machine ever built — a car engine, a bicycle, a pair of scissors — is, underneath, some combination of these six basic devices working together.
Simple machines don't reduce the total work needed — that would break the conservation of energy explored in the previous article. What they actually do is let you trade force for distance, or the other way around.
A lever: pushing down with a small force over a long distance on one side lifts a much heavier load a shorter distance on the other side. The work done on each side is the same — only force and distance have been traded.
A ramp is the clearest example: pushing a heavy box up a long, gentle ramp takes less force than lifting it straight up — but you push it over a much longer distance to reach the same height. Multiply the (smaller) force by the (longer) distance on the ramp, and it comes out to almost the same total work as lifting it straight up with a (larger) force over a (shorter) distance. The machine made the task easier on your muscles, not smaller in terms of physics.
A wheelchair ramp rising 1 metre over a 10-metre length requires roughly one-tenth the force to push a wheelchair up compared to lifting it straight up 1 metre — but you have to push it ten times farther. The work done is approximately the same either way; the ramp simply changes how that work gets applied.
Most tools you actually use are combinations of these six machines, not a single one in isolation.
| Everyday Tool | Simple Machines Combined |
|---|---|
| Bicycle | Wheel and axle, lever (pedals) |
| Scissors | Two levers joined at a fulcrum, with wedge-shaped blades |
| Can opener | Lever, wheel and axle, wedge |
| Car jack | Screw, lever |
Ancient Egyptian builders are widely believed to have used ramps and levers to move the multi-tonne stone blocks used to build the pyramids, since neither the wheel and axle nor powered machinery as we know them today were available at the time. No single worker could lift a multi-tonne block — but by trading force for distance across a long ramp, and using levers to reposition blocks, a large team could move them into place using tools that were, in physics terms, remarkably simple.
Just like the energy converters explored in the previous article, no real simple machine is perfectly efficient — friction between moving parts always converts some of the input work into wasted heat. A well-oiled pulley loses very little energy to friction; a rusty, poorly maintained one loses considerably more, meaning more of your effort disappears as heat rather than lifting the load.
Work and power sound like everyday words, but in physics they mean something exact: work is force causing movement, and power is how fast that work gets done. Simple machines don't cheat these rules — they can't — but by trading force for distance, they've let humans move stone blocks, raise water, and split wood using nothing but cleverly shaped tools, for thousands of years before electricity or engines ever existed.
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