Pressure in liquids: why deep water pushes harder
Physics · Key Stage 3 · The School
Why depth increases pressure
At any depth in a liquid, the pressure comes from the weight of all the liquid sitting above that point. Go deeper, and there is more liquid above you. So there is more weight pressing down, and pressure goes up. This is why a dam is built thicker at the bottom than at the top. Down there, the wall must hold back far more pressure.
Pressure pushes every way at once
Liquid pressure does not just push down. It pushes sideways too, and even upward, equally in every direction at a given depth. That is why a balloon pushed underwater feels squeezed from all sides, not just from above. It is why a submarine's hull must be strong all the way round, not just on top.
Air pressure works the same way
The air above us has weight too. It presses down in exactly the same way a liquid does. This is called ATMOSPHERIC PRESSURE. It is strongest at sea level, where the whole depth of the sky sits above you. It drops as you climb a mountain, because there is less air above you pressing down.
Your ear has a small air-filled space behind the eardrum. Normally the air pressure inside that space matches the pressure outside it. So the eardrum sits flat and comfortable. Now change height quickly — diving underwater, or a plane taking off. The OUTSIDE pressure changes fast. The pressure trapped inside your ear does not keep up right away. That mismatch pushes or pulls on the eardrum, and that is the discomfort you feel. Swallowing or yawning opens a tiny tube linking the ear to the throat. Air flows through it and the two pressures even out. That is the real cause of the "pop" that brings relief.