Since the molecules are in continuous motion, they are always colliding with other molecules or the solid surfaces of their con­tainer. In a perfect gas, all these collisions are taken to be per­fectly elastic i. e. when a molecule strikes a solid surface, the surface experiences a force equal and opposite to the time rate of change of momentum of the rebounding molecule. This force causes the gas to exert an overall pressure on the container or other immersed body.

This force per unit area is defined as the pressure, the units in the SI system being Pa (Pascals) 1 Pa = 1 N/m2. In a fluid at rest the pressure acts normally to the solid surface.

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