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Have you ever wondered how cranes can lift such heavy things? Their pulleys really help lighten the load. In this activity, have a play with some pulleys. These simple machines can make heavy weights as light as a feather!

You will need

• Several sewing machine bobbins (can be purchased at sewing stores)
• Japanese-style chopstick
• String
• Scissors
• Water bottle with handle, or an empty 2 L milk carton
• Two tables that are the same height
• Someone to help

The simplest tackle

1. Put two cups of water into the water bottle.
2. Get help to put the two tables close together, so there is a gap of about 5 cm between them.
3. Put one bobbin onto the chopstick.
4. Place the chopstick so it bridges the gap between the tables, with the bobbin in the gap.
5. Cut a piece of string about two metres long, and tie one end of the string to the water bottle.
6. Put the water bottle directly under the chopstick, and then run the string up and over the bobbin.
7. Get your assistant to hold the chopstick so it doesn’t move.
8. Pull down on the string. This will lift the water bottle off the ground!

More pulleys

1. Untie the long string from the bottle.
2. Cut another piece of string, around 10 cm long.
3. Thread a bobbin onto the short string, and then tie it in a loop around the handle of the water bottle. Knot it in place.
4. Tie one end of the long string on the chopstick, and make sure there’s a bobbin on it too.
5. Put the chopstick over the gap between the tables and put the water bottle underneath.
6. Run the long string through the bobbin on the bottle, and then over the bobbin on the chopstick.
7. Get your assistant to hold the chopstick so it doesn’t move.
8. Pull down on the string. Was it easier to lift the bottle this time? Did the bottle move as fast?

Going even bigger

You can add more bobbins to make your pulley more powerful. For a three bobbin system, tie one bobbin to the bottle and put two on the chopstick. Tie the long string to the bottle, then thread it around one chopstick bobbin, through the bottle bobbin and finally the second chopstick bobbin. Before you lift the bottle, make sure your string isn’t tangled.

A four bobbin pulley set has two bobbins on the chopstick and two on the bottle. Tie the long string to the chopstick, not the bottle. Can you work out how to rig it? You can keep adding more and more bobbins, but it might get tricky to prevent tangles!

What’s happening?

In this activity, you’re making a simple machine called a pulley set, or a block and tackle. These machines are often used to lift things. They can be found in lots of places, from cranes and sailboats to elevators and exercise machines.

Pulleys aren’t just for show. A block and tackle can make heavy things easier to lift, as they make a small force stronger. With a big enough block and tackle, you could use a small rock to lift a boulder!

But you don’t get energy for free. In this activity, as you add pulleys, the bottle moves slower. There’s a mathematical rule that goes with this. If you halve the force to move something, then you double the distance you pull the rope.

In the end, it uses exactly the same amount of energy to get the load moved. So if you want to lift a boulder with small rocks, you’ll need a lot of small rocks (or one rock falling a very long way)!

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