What it shows
An object in water feels lighter. The upward force, the buoyant force, equals the weight of the water it pushes aside, and the rubber band shows it by shrinking.
The science
A rubber band stretches in proportion to the force that pulls it, so its length shows weight. When the stone is lowered into water, the water pushes up on it with a buoyant force. By Archimedes’ principle, this force equals the weight of the water displaced. The band shortens by an amount that matches the weight of the overflow water. A denser object displaces the same volume of water as a lighter one of the same size, so the buoyant force depends on the volume that is under water, not on the weight.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A thick rubber band
- A stone with a thread loop, about 100 g
- A stand or a nail on a plank to hang the band
- A plastic bottle cut into an overflow can with a spout
- A small cup for the overflow
- A ruler
- A kitchen scale
- Water
Safety
For this project: An adult must cut the bottle and cover the sharp edge with tape. Wipe up spills to avoid slipping.
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.
Hang the rubber band from the nail and hang the stone from the band. Measure the length of the band, which shows the weight in air.
Fill the overflow can with water until it just stops dripping from the spout. Place the empty cup under the spout.
Lower the stone slowly into the can until it is fully under water but not touching the bottom. Measure the band length again.
Collect all the water that overflows in the cup. Weigh the water on the scale, remembering 1 g of water is 1 ml.
Calibrate the band: hang known masses of 20 g, 50 g and 100 g and make a graph of length against mass.
Use the graph to convert the length change into grams-force.
Compare the loss in weight of the stone in water with the weight of the water that overflowed. They should be close.
Repeat with a stone of different size and a block of wood held under the water by hand.
What you should see
The loss of weight of the stone in water is about equal to the weight of the displaced water, within a few grams, which shows Archimedes’ principle.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1State Archimedes’ principle.
The upward force on an object in a fluid equals the weight of the fluid it displaces.
2Why does a stone feel lighter in water?
The buoyant force pushes upward and reduces the pull on your hand.
3Why does a ship float although it is made of steel?
Its shape displaces a large volume of water whose weight equals the weight of the ship.
4What happens to the buoyant force if the stone goes deeper?
It stays the same, as long as the stone is completely under water.
Take it further
Use the method to find the density of the stone by dividing its weight in air by the loss of weight in water.
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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