Have you ever felt pressure on your body– especially in your ears– as you swim deep underwater? The deeper you go, the more pressure you feel. This is the core of a principle called buoyancy. Buoyancy explains why, in a liquid, what goes down just might come up!
Let’s think about how water affects this metal sphere. Pressure increases with depth, so the water at the bottom of the sphere pushes it up more than the water at the top pushes it down. The same goes for you – when you float vertically in the water, the pressure is always greatest at your feet. This unbalanced force, called buoyancy, is illustrated here:
Buoyancy doesn’t care how heavy things are, just how much space they take up in the water. The buoyant force lifting an object up is always the same as the weight of the liquid that would normally occupy that space. If you push a beach ball underwater, the buoyant force pushes back with the weight of the water that the beach ball has shoved out of the way. Push a yoga ball under, and you’ll feel a push back equal to– you guessed it– the weight of a yoga ball full of water!
So, why do some things float and others sink? We can think of the answer as a battle between the forces of buoyancy and gravity. A rock is heavier than the amount of water it pushes out of the way. Gravity pulls down more than buoyancy pushes up, so the rock sinks to the bottom. A beach ball, on the other hand, is much lighter than a beach-ball-sized blob of water, so in this case buoyancy beats gravity. It floats! In general, if an object is less dense than water, it floats. If it’s more dense, it sinks.
Buoyancy works in other substances, too. A helium balloon floats in air for the same reason a beach ball floats in the pool. Water stays in the pool because it’s more dense than air– if it wasn’t, it would float to the ceiling!