Floating and sinking — Physics, 7–10 years
Whether an object floats depends on its weight, its size and the water it displaces. A heavy-looking object can float if its shape includes enough air.
What floating means
Water pushes upward on objects placed in it. An object floats when this upward push can support its weight; it sinks when its weight is greater than the support. Shape matters because a wide, hollow shape pushes aside more water than a solid lump of the same material.
Why people needed this idea
People needed boats to carry people and goods across rivers and seas. A rule based only on material would fail: iron can sink as a lump but float as a boat. Studying floating explains how to choose a shape that pushes aside enough water to support a load.
A foil boat
Make two shapes from equal pieces of aluminium foil. Crush one into a tight ball and make the other into a wide bowl. Put both gently in water: the ball sinks because it pushes aside little water, while the bowl floats. Add coins one at a time; if the bowl holds 12 coins before sinking, its shape supports that load but not the next one.
A common mistake
It seems that heavy things should always sink and light things should always float. That guess works for many small objects, so it feels sensible. But weight must be compared with the upward support from displaced water: a huge steel ship floats, while a small steel marble sinks.
Where it is used
Boat builders shape hulls so boats displace enough water to carry passengers, cargo and engines. Life jackets contain light foam and help a person stay near the surface. Engineers also test floating platforms, buoys and submarines by controlling their volume, weight and the water they displace.
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