Hydrostatic Pressure Force "must" be perpendicular

astromajor

Hi, I'm taking AP Physics B and we started learning about fluids and pressure. My teacher told me that the pressure of a fluid acting in a a container only applies a perpendicular force to the sides, which doesn't make sense. If you 'zoomed in', molecules could strike the surface of the container at any angle, right? So couldn't the forces be in anything from parallel (well, not really) to perpendicular, and everything in between? Is it just a simplification or literally only the molecules that hit the surface perpendicularly that impart a force? Thanks for your time!

Nugatory

Neither. It's not a simplification and it's not the case that only the perpendicular-hitting molecules impart a force. There are a very large number of molecules involved, so that on average, for every molecule that strikes from one side of perpendicular another molecule has just struck or is about to strike from the other side of perpendicular. Thus, the non-perpendicular components of each impact cancel out on average, leaving only the perpendicular components to impart a force.