• The force acting on a body perpendicular to its surface is called thrust. • Thrust is the total force acting on the surface of the body. • Thrust per unit area is pressure
Pressure = Thrust
S.I unit of pressure is N/m2 or Pascal
EVERYDAY OBSERVATIONS RELATED TO PRESSURE:
• School bags have wide straps so that the weight of the bag falls over a large area of the shoulder producing less pressure on the shoulder. Due to this less pressure, it is comfortable to carry the heavy schoolbag.
• A sharp knife cuts well than a blunt knife because the edge being very thin, the force of our hand falls over a very small area of the object, producing a large pressure. And this large pressure cuts the object easily. On the other hand the blunt knife has a thicker edge due to which the force of our hands falls over a large area producing lesser pressure.
• The tip of a thumb pin is sharp so that due to its sharp tip, the pin may put the force on a very small area of the wooden board producing a large pressure sufficient to pierce the board.
While bathing we notice that the mug of water suddenly appears heavier as soon as it comes above the water surface.
Similarly, when a fish is pulled out of water, it appears to be heavier in air than inside the water. All objects whether completely immersed inside a liquid (or fluid) or partially inside, appear to be lighter than in air.
Objects appear to be less heavy in a fluid or in any liquid because the liquid or water exerts an upthrust on the objects immersed in it.
The upward force exerted by a fluid on an object when it is partly or wholly immersed in it is called buoyant force. The Buoyant force is also known as the upthrust.
(a) A stone is weighed in air
(b) The stone weighs less when immersed in a liquid because of the buoyant force exerted by the liquid on it in the upward direction....