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Why do things float or sink?

Anchor on the seabedAnchor on the seabedWhy do some things float and other things sink? It depends on the density of the object. An object is made of substances that are themselves made up of molecules. Density is a measure of both how massive those molecules are, and how closely packed together they are. If the object's total density is greater than that of water, it sinks, but if it is less, it floats.

Floating in water

A cork is less dense than water, but a stone the same size is denser. The cork floats but the stone sinks. Rubber rings float because they are full of air—which is less dense than water. People are almost the same density as water, which is why you just about float. Air-filled buoys are attached by a chain to a heavy weight on the sea bed that stops the buoy floating away.


Things floating or swimmingThings floating or swimming 

The buoyancy force acting on a shipThe buoyancy force acting on a ship

How do ships float?

Boats and ships—especially enormous steel-hulled ships—look too heavy to float. But because the ship is hollow it contains a lot of air. It is this air that makes the ship, overall, less dense than water, so it floats. The force of the water pushing upwards against something is called the upthrust, or bouyancy. When something floats, the upthrust exactly matches the force of gravity, which pulls the object down.


Hot-air balloonHot-air balloon

Floating in air

Floating in air is the same as floating in water. Anything that is less dense than air will float in it. When you heat air it becomes less dense because its molecules spread out. Hot-air balloons use this principle to fly. Air inside the balloon is heated, which makes it less dense than the cooler air outside. This causes the balloon to rise up into the air. 

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Why do ships float?

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Gold is nearly 20 times the density of water. But it is not the densest element. That is osmium.

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As liquid water freezes into solid ice, it expands and becomes less dense. This is why ice floats in liquid water.

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Saturn is the least dense of all the planets: if a large enough bathtub could be found, it would float in the water.

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