A static fluid presses on the inside wall of its container. In which direction does the force produced by that pressure act?
AAlways perpendicular to the surface
BAlways parallel to the surface
CAlways downward under gravity
DAlways along the flow direction
Answer & Solution
Correct answer: A. Always perpendicular to the surface
1. A force acting parallel to a surface is a shearing force.
2. A static fluid can neither withstand nor exert shearing forces.
3. The only remaining possibility is a force at right angles to the surface, so pressure forces are always perpendicular to any surface they act on.
4. Gravity acts downward on the fluid, but the pressure force on a side wall is horizontal, so downward is wrong.
5. A static fluid is not flowing, so there is no flow direction to follow.
_Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.3 Pressure_
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