What does this sentence mean?- liquid surface is always perpendicular to the net force

Answer 1

Probably it means the following if a liquid is in a static state : if a force applied to the liquid surface would have a longitudinal component, e.g in x -direction , the surface layer of liquid would have to start accelerating toward the same direction. That would contradict with the assumption that the liquid is in a static state.

This is quite similar to the case of electrostatic field on the surface of a conductor in a static state: the field must be normal (perpendicular) to the surface, otherwise it's longitudinal component would push electrons along the surface i.e. creating electric current. But we assumed the conductor to be static (i.e. no currents)...

Hope it clarified your question

Dr.Ariel B.