What it shows
A single fixed pulley does not make a load lighter, but it changes the direction of the pull, so you lift a load up by pulling down. That is why cranes, flagpoles and wells use pulleys.
The science
A pulley is a wheel with a groove that a rope runs over. When the pulley is fixed to a frame, pulling the rope down makes the load on the other side go up. The force you need is about the same as the weight of the load, so there is no saving in effort, only a change in direction, which is often more comfortable because you can use your body weight. Friction in the pulley adds a little to the effort.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An empty thread spool (free from a tailor)
- A pencil or a skewer to run through the spool
- A cardboard box or two chairs as the frame
- About 1 metre of string
- A small bucket made from a paper cup with a string handle
- Pebbles or coins as the load
- A rubber band to use as a simple force meter
Safety
For this project: Keep small pebbles and coins away from young children, and an adult must make the holes for the pencil.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Push the pencil through the spool and rest the ends in two holes cut near the top of the cardboard box, so the spool spins freely.
Lay the string over the spool so it sits in the groove, and tie the paper cup bucket to one end.
Hook the rubber band to the free end of the string so you can pull on the band and watch how far it stretches.
Put five pebbles in the bucket and pull down on the rubber band until the bucket rises; mark the stretch of the band on a strip of paper.
Now lift the same bucket straight up by the rubber band without the pulley and mark the stretch again.
Compare the two marks: they should be almost the same, which shows the fixed pulley does not reduce the force.
Repeat with ten pebbles and notice that the band stretches further in both cases.
Decorate the box as a crane cab and add a cardboard jib so the pulley hangs over the edge like a real crane.
What you should see
The crane lifts a cup of pebbles when you pull the string downwards. The rubber band stretches by about the same amount with and without the pulley, showing that a fixed pulley changes only the direction of the force.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Does a fixed pulley make the load easier to lift?
No, the effort is about the same as the load; it only changes the direction of the pull.
2Why is pulling down easier for a person than lifting up?
You can use your own body weight and your strong leg muscles when pulling downwards.
3Name two places a fixed pulley is used.
A well with a rope and bucket, and a flagpole.
4What does the rubber band measure?
The pulling force: the more it stretches, the bigger the force.
Take it further
Add a second spool that hangs from the string and moves with the load, and show that the rubber band now stretches about half as much.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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