Pressure and pressure differences in fluids (physics only)

Particles in a gas or liquid move randomly in every direction, and their collisions with the walls of a container produce a force on the container — this is felt as pressure. Pressure always acts at right angles (normal) to a surface.

Pressure in a liquid increases with depth (a taller column of liquid above a point has more weight pressing down) and with density. A submerged object experiences greater pressure on its lower surface than on its upper surface (because the lower surface is deeper) — this pressure difference produces a net upward force called upthrust.

An object floats if its weight is less than or equal to the upthrust it experiences once fully submerged — equivalently, if its weight is less than the weight of fluid it would displace if fully submerged. A ping pong ball floats because its density is lower than water's: the weight of water it displaces is greater than its own weight, so the resultant force is upward (buoyant).

The Earth's atmosphere is a relatively thin layer of air surrounding the planet. Atmospheric pressure at a given point results from the weight of all the air above that point pressing down.

Air density decreases with increasing altitude — there is progressively less air above you as you go higher. Because there is less air (and so less weight of air) above a given area at high altitude than at low altitude, atmospheric pressure decreases as altitude increases.