• Thrust

    The force acting on an object perpendicular to the surface is called thrust. The thrust is a vector quantity and is measured in the units of force i.e., Newton (N) and kilogram-force.



    Pressure is defined as the thrust acting per unit area of a body. Mathematically,

    Pressure  = Thrust/ Area

    Pressure is measured in SI in Newton per square metre (Nm-2) or Pascal (Pa). It is also measured in bar and millibars. 1 bar = 105 Nm-2.


    Applications of the Concept of Pressure

    The army tank has a large weight. Therefore to avoid large pressure on the ground its weight is distributed throughout the tank. This is done by making the tank run on a steel track than on wheels. The steel tracks reduce the pressure of the ground.

    A school bag has wide straps made of thick canvas. This is done to distribute the entire weight of the bag over a larger area. This, in turn, will produce a small pressure.

    The tip of a needle is made sharp so that the force applied to it acts on a small area. This increases the pressure on it and hence it can easily pierce the cloth.

    The blades of knives are made sharp so very small surface area and on applying force, it produces large pressure and cuts the object easily.

    Tyres of trucks or motorbuses are made wider to avoid large pressure on the ground.

    A nail has a pointed tip so that the force applied to it increases the pressure on the pointed tip and hence it can easily pierce the wood/wall.

    Buildings have wide foundations so that its weight distributed on large surface area.


    Pressure In Fluids

    All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight. Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed. The pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.

    The pressure at all points and in all directions in a liquid is the same. It increases with the depth of the fluid. When a body says a block, is immersed in a liquid the pressure experienced by it is found to be greater at its bottom surface than its top surface. The difference in this pressure is upthrust. Thus, buoyancy is caused by differences in pressure acting on opposite sides of an object immersed in a static fluid. Pressure variations in a fluid are typically caused by gravity (since P = P0 + ρgh), but in general buoyant forces act opposite the direction of the frame of reference acceleration. Under conditions of apparent weightlessness, there can be no buoyant forces.