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If you have done some of the other
experiments together as a class, the children should be able to work
on this in groups.
When a ship floats it is said to have positive buoyancy. When it sinks it has negative buoyancy. When it neither sinks to the bottom or rises to the surface, it is said to have neutral buoyancy (like a deep sea diver).
When you place a piece of wood in a pail of water, the wood displaces some of the water, and the water level goes up. If you could weigh the water that the wood displaces, you would find that its weight equals the weight of the wood.
This doesn't mean that if you had a few blocks of wood that were exactly the same size and shape, they would each displace the same amount of water. A block of wood made of oak, for example, sits deeper in the water (and therefore displaces more of the water) than does a block of pine. The reason is that it's heavier for its size, or denser -- in this case, the molecules that make it up are more closely packed together than the molecules that make up the pine.
Materials needed
* nails (large heavy ones) * polystyrene bits * modelling clay * containers of water
Directions
Try to make the nail neutrally buoyant.
1. Put the nail in the water - you will see that it sinks to the bottom.
2. Put a chunk of the polystyrene in, and it will float on the surface.
3. What would happen if we attached some polystyrene to the nail? Could we get it to float under water, without sinking to the bottom or rising to the top?
4. Experiment with adding bits of polystyrene until you can get the nail to float below the surface.