Why do heavy things sink?

Heavy things sink in fluids (like water or air) because of density. Density is a measure of how much mass an object has for its size. The more mass an object has, the denser it is.

Fluids also have density. For example, water has a density of 1 gram per cubic centimeter (g/cm³). This means that for every cubic centimeter of water, there is 1 gram of mass.

When you put an object in a fluid, the object will displace an amount of fluid that is equal to its own volume. For example, if you put a 1 cm³ block of wood in water, it will displace 1 cm³ of water.

If the object is denser than the fluid, it will displace an amount of fluid that is less than its own mass. This means that the object will have more mass than the fluid it displaces, and it will therefore sink.

For example, if you put a 1 cm³ block of iron in water, it will displace only 0.78 cm³ of water. This means that the iron block will have more mass than the water it displaces, and it will therefore sink.

If the object is less dense than the fluid, it will displace an amount of fluid that is greater than its own mass. This means that the object will have less mass than the fluid it displaces, and it will therefore float.

For example, if you put a 1 cm³ block of cork in water, it will displace 1.25 cm³ of water. This means that the cork block will have less mass than the water it displaces, and it will therefore float.