What it shows
A movable pulley hanging with the load shares the weight between two lengths of string, so you need only about half the force to lift it, but you must pull twice as much string. Adding pulleys trades distance for force.
The science
In a block and tackle the load hangs from a pulley that moves with it, and the string that supports that pulley runs up over a fixed pulley to your hand. Two lengths of string now hold the load up, each carrying about half its weight, so your pull is only half the load. Nothing is free: to raise the load by 10 cm you must pull 20 cm of string, because both supporting lengths have to shorten. Friction in the spools means the real saving is a little less than half.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two empty thread spools and two pencils or skewers
- A wire coat hanger or a wooden frame to hold the top spool
- About 2 metres of string
- A water bottle half full as the load
- A rubber band or a spring balance to compare the pull
- Two bent paper clips or wire hooks
- A ruler
Safety
For this project: An adult must bend the wire hooks, and the load should be lifted only a little way above a soft surface.
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.
Thread a pencil through the first spool and tie it to the hanger so the spool hangs under the frame and turns freely; this is the fixed pulley.
Thread the second pencil through the second spool and tie a wire hook under it so the bottle can hang from this movable pulley.
Tie one end of the string to the frame next to the fixed pulley, pass it down under the movable spool, back up over the fixed spool and let the free end hang down.
Hang the bottle from the hook on the movable pulley and tie the rubber band to the free end of the string.
Pull the rubber band slowly until the bottle rises and mark how far the band stretches on a paper strip.
Now hang the bottle directly from the rubber band without any pulley and mark the stretch; it should be about twice as much.
Raise the bottle exactly 10 cm with the tackle and measure how much string you pulled through; it should be close to 20 cm.
Present the two stretch marks and the two distances side by side and explain the trade between force and distance.
What you should see
The bottle rises with roughly half the pull needed to lift it directly, while about twice the length of string passes through your hand. The rubber band marks make the halving easy to see.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is the effort about half with the movable pulley?
Two lengths of string share the load, so each carries about half of it.
2What is the price of the smaller force?
You must pull twice as much string to lift the load the same height.
3Why is the saving a little less than half in practice?
Friction in the spools and the weight of the lower pulley add to the effort.
4Where are block and tackle systems used?
On cranes, sailing boats and for lifting engines in garages.
Take it further
Add a third spool and work out how many string lengths support the load and what fraction of the effort is now needed.
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.
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