Fluid Systems
When fluid molecules are in a container, they constantly collide with the walls. While random collisions create forces in all directions, the parallel forces cancel out, leaving only the perpendicular forces. This means fluid pressure always acts perpendicular to any surface it contacts.
A stationary fluid is in equilibrium, with forces balanced at all points. Imagine two points in a container of liquid—point A near the top and point B deeper down. Point B experiences greater pressure because it must support more fluid weight above it than point A does. This explains why you feel increasing pressure as you swim deeper in water.
Did you know? The pressure you feel at just 10 feet underwater is noticeably greater than at the surface—this is why your ears might "pop" when diving into a pool!
Both liquids and gases are considered fluids, though they behave differently. Liquids are typically "incompressible" (they maintain their volume when pressured) while gases are "compressible" (their volume changes under pressure). This distinction becomes important when analyzing how fluids interact with other objects.
