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Pressure

Pressure of fluid molecules

In the animation at the previous page we may have noticed that the molecules collide with the wall of the vessel in all possible directions. Nevertheless, the momentum transfer in every collision is always perpendicular to the wall. The momentum transfer in each collision results in a force on the wall, the reaction force to the force the wall has to give to the molecule in order to to change its direction (not its speed by conservation of mechanical energy). Indeed, Newton's law of inertia says that an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force, whereas Newton's third law states that for every action there is an equal and opposite re-action.

As a consequence the wall expierences a force perpendicular to it which results from the countless number of collisions per second with the molecules of the fluid. The sum of the forces F of all the molecules impacting on the wall divided by the area S of the wall is defined to be the pressure P:

P = F/S

Pressure has units

units of pressure = force/area = N/m² = Pa (Pascal)



fluid pressure