A liquid at rest in a container exerts a force on the walls and bottom of the container. The force is exerted due to collisions of atoms / molecules of liquid with the walls of the container. In each collision, there in a change in linear momentum of atoms / molecules of the liquid. Conesquently, the wall exerts a force on atoms / molecules and vice–versa.
For a liquid at rest; the force exerted by the liquid is always normal to the area of surface in contact. If the force F is in any other direction than normal as shown in Fig. 1; we resolve it into rectangular components (1) F cos θ along surface and (2) F sin θ perpendicular to surface. The component F cosθ will make liquid particles move along the surface, i.e. liquid is not at rest. For liquid to be at rest F cos θ = 0 or θ = π/2 i.e. force F acting on liquid surface must be along the perpendicular (i.e. normal) to the surface.
The total force exerted by the liquid normal to the surface in contract is known as the thrust of liquid on the surface.
The normal thrust (or force) exerted per unit area of the surface in contract with liquid, is the pressure (P) exerted by the liquid on the surface i.e. p = F/A. SI Unit of pressure is Nm–2 (≡ Pa).
Force is a vector quantity but pressure is a scalar quantity. The direction of pressure at any point is always perpendicular to a small area around the point considered. Pressures add up algebraically. Also pressure is transmitted equally in all directions; therefore no direction can be associated with pressure.
