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Science project idea

Why does a clay ball sink but a clay boat float?

The same lump of clay sinks as a ball but floats as a bowl, because the bowl shape pushes aside much more water. Floating depends on the shape and the water displaced, not only on the weight.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Physics.Browse PhysicsAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

The same lump of clay sinks as a ball but floats as a bowl, because the bowl shape pushes aside much more water. Floating depends on the shape and the water displaced, not only on the weight.

The science

When an object is put in water, the water pushes up on it with a force called buoyancy. This upward push is equal to the weight of the water the object pushes out of the way. A tight clay ball pushes aside only a little water, so the push is less than its weight and it sinks. Spread into a wide bowl, the same clay takes up much more space in the water, pushes aside more water and gets a bigger upward push, enough to hold it up. Steel ships float for the very same reason.

Materials

Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A lump of modelling clay or plasticine about the size of a lemon
  • A tub or bucket of water
  • A handful of coins or marbles as cargo
  • A ruler
  • A towel for spills

Safety

This project uses safe, everyday materials. Still, read the rules below with a parent before you start.

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 rules

Step 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.

  1. Roll the clay into a tight ball and drop it gently into the water; it sinks straight away.

  2. Take the ball out, dry it and press it into a wide, shallow bowl with thin walls.

  3. Lower the bowl onto the water and see it float.

  4. Add coins one at a time to the floating bowl and count how many it holds before it sinks.

  5. Make the bowl wider and taller with the same clay and test again; a bigger bowl usually carries more coins.

  6. Try a tall narrow cup shape and a flat raft shape and compare how many coins each carries.

  7. Watch the waterline on the bowl as you add coins and notice it sits deeper each time.

  8. Record each shape, its size and the number of coins carried in a table.

What you should see

The clay ball sinks while the clay bowl floats and carries several coins. Wider bowls with higher sides carry the most coins before going under.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1The ball and the bowl weigh the same, so why does only one float?

The bowl pushes aside much more water, so the water pushes up on it harder.

2What is the upward push of water called?

Buoyancy, or the upthrust.

3Why do heavy steel ships float?

Their hollow shape pushes aside a weight of water equal to the ship’s weight.

4What happens when a boat is overloaded?

It sinks lower until water comes over the sides and it goes under.

Free ebook: Viva Questions for Science Projects

Take it further

Float the bowl in salt water and in fresh water and compare how many coins it holds in each.

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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