Unit 6, lesson 28. About 15 minutes.

Pulleys, ramps, wheels and gears

How did people lift huge stones thousands of years ago with no engines?

Play

Getting the experiment ready…

Explain

Ropes and wheels make lifting easy.

Ramps make going up easy.

Simple machines make jobs easier. They include pulleys, ramps, wheels and gears.

With more ropes holding a load, each rope only has to hold a part of it. So you pull with less force.

But there is a catch: you must pull more rope. A ramp works the same way: less force, but a longer path.

Pulley systems share a load among n supporting ropes, so F = W / n.

Energy is still conserved. The input work F × d must equal the output work W × h, so distance goes up by n.

Worked example: lifting 400 N with 4 ropes takes 100 N, but you pull 4 m of rope for a 1 m lift.

Use a simple machine
Force gets smaller
Distance gets longer

Want the full story?

Real pulleys have friction and weight, so the force is a bit more than W / n. Gears are wheels with teeth. A small gear turning a big gear gives more turning force and less speed, and bike gears use this.

The Formula

pull needed = weight ÷ number of supporting ropes

F = \frac{W}{n}

$F$: the pull needed, measured in newtons (N)
$W$: the weight lifted, measured in newtons (N)
$n$: the number of ropes holding up the load, which has no unit

Try the numbers

Weight: 400 NRopes: 1

Pull needed: 400 N

Try it at home

You need a big book or board, a low step, a toy car.

You need: a big book or board, a low step, a toy car.

Push a toy car up a ramp made from a book.

Lean a board or big book on a low step to make a ramp.
Lift a toy car straight up to the step.
Now push it up the ramp, and compare the effort.

What's happening: the ramp takes less force, but the car has to travel further.

Physics is life

Your body

Your knee has a pulley.

Your kneecap works a bit like a pulley, changing the pull of your thigh muscle.

Daily life

Bikes have gears.

Bike gears, cranes and lifts all use simple machines.

Nature

Roads wind up mountains.

Roads zigzag up mountains, as a long, gentle ramp is easier than a steep one.

Space

Rovers have gears on their wheels.

Mars rovers use gears to turn their wheels with great force.

Quick check

Question 1 of 3

How does a pulley help?

Words to know

Pulley: A wheel and rope that make lifting easier.

Pulley: A wheel and rope that make lifting easier.
Simple machine: A tool that changes the size or direction of a force.
Work: Energy transferred when a force moves something.

Next up

Turning forces: moments