Physics · Mechanical Properties Of Fluids · NEET
Gravity pulls the fluid down, so students think pressure acts only downwards. But pressure is caused by molecules pushing on every surface they touch, from all sides. The water pushes on the side walls of a tank too - that is why a dam wall must be thick at the bottom. At one point, the push is equal in all directions.
Take a tiny wedge-shaped fluid element (a right-angled prism) at one depth. Since it is so small, gravity is nearly the same over it and its weight is negligible compared to the pressure forces. Each face has a force = pressure x area, always normal (perpendicular) to that face. For the element to stay at rest, the horizontal and vertical force components must each cancel. Working this out gives Pa = Pb = Pc, so pressure is the same on all three faces, hence in all directions.
Pressure is a scalar. It has no direction of its own. The force on a surface has direction (always normal to the surface), but the pressure value at a point is just a number. This is exactly why it can be the same in all directions - a scalar cannot point one way.
In a fluid at rest there is no sliding or shear force. The only force a static fluid can exert on any surface is perpendicular (normal) to that surface. If there were a sideways (tangential) force, the fluid layers would slide and the fluid would not be at rest. So every pressure force points straight into the surface.
No. The rule is for a fluid at rest (static). When a fluid flows, viscous and shear forces appear, and pressure can differ with direction. For NEET hydrostatics problems, always assume the fluid is at rest unless the question says it is flowing.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because the fluid is at rest. If the push were larger on one side of a tiny element, the net force would make it move, contradicting 'at rest'. So all sideways forces balance and pressure is equal in every direction.
Pressure is a scalar. It has magnitude but no direction. The force it produces on a surface is normal to that surface, but the pressure value itself has no direction.
Yes. A fluid pushes on the bottom and on the side walls, always perpendicular to the wall. That is why deep water pushes hard on the lower part of a dam or tank wall.
Equal-in-all-directions is true at a single point. Depth-variation compares two different points. Deeper points carry more fluid weight above them, so P = P0 + rho g h, but at each point the value is still the same in every direction.
Yes. Air and other gases at rest also exert equal pressure in all directions at a point. This is why a balloon inflates evenly in every direction.
