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The general term ‘fluid refers to any substances that has no definite shape and has the ability to flow. Liquids and gases do not have any definite shape and they flow. Thus liquids and fluids. Many areas of science and industry are concerned with fluids. Engineers and scientists who are engaged in the design of submarines, aeroplanes, missiles, rockets, ships and satellites are concerned with fluids. Devices used in science and industry, such as barometers, hydraulic presses, hydraulic brakes, etc. are manufactured by taking advantage of the properties of fluids.
We will study some basic properties of a fluid at rest and in motion.


Unlike solids, fluids do not have a definite shape; they take the shape of the vessel in which they are contained. We have seen that solids can withstand a tangential force and can be sheared. But fluids cannot withstand any force tangential in their surface. This can be understood as follows:

Suppose you are sitting in a boat floating on water in a lake and the boat and the water are still, i.e. they are in a state of static equilibrium. The forces acting vertically downwards on the system are: (i) the weight of the atmosphere, (ii) your weight, (ii) the weight of the boat, and (iv) the weight of the water in the lake. From Newton’s third law the bottom of the lake exerts an upward forces equal to W so that the system (i.e. the boat, you and water) is in equilibrium.

Now suppose a small horizontal force (parallel to the water surface) is applied to the boat. This force can be applied by a person on the shore by pulling at the rope tied to the boat. You can also exert such a force sitting in the boat by throwing something (say fish) in a direction parallel to the water surface. The fish will, in turn, exert an equal and opposite force on you and the boat. You can also exert such a force sitting in the boat by throwing something (say fish) in a direction parallel to the water surface. The fish will, in turn, exert an equal and opposite force on you and the boat. You will notice that a small force F is enough to move the boat. If you look at the water near the boat, you will notice that water also moves in the direction of the boat. The system is no longer in equilibrium. This if a tangential force is applied to a liquid, it begins to flow, since it cannot withstand any tangential force. Thus we conclude that the forces acting on a fluid in equilibrium are always perpendicular to its surface. It is for this reason that the tree surface of a fluid at rest in a container is always horizontal.

Since the direction of the force acting on a fluid in equilibrium is always specified (being normal to the surface). The force is completely described by its magnitude. Hence, while dealing with fluids, it is more meaningful and convenient to use the concept of pressure rather than force. The total force acting on a surface is called thrust. Pressure is defined as the trust exerted normally on a unit area of the surface. In the SI system the unit of pressure is Newton per square metre (N m-2) also called Pascal (symbol Pa).


Since pressure is force per unit area it follows that a given force will exert a different pressure if it acts over a different area. To clarify further , let us first consider the example , a brick. Figure shows a brick of dimensions 20 cm × 10 cm × 5 cm lying on a table. In the first case the brick is lying on its side and in second case it is standing on its end. If the mass of the brick is 5 kg it exerts a force of 5kg x 9.8 ms-2 =49 N on the surface. Here 9.8 ms-2 is the acceleration due to gravity.

In the first case the force of 49 N acts over an area= 20 cm x 10 cm = 200 m2 = 0.02 cm2. Hence it exerts a pressure of

In the second case the same force of 49N now acts over an area = 10cm x 5cm = 50 cm2=0.05m2. Hence it exerts a pressure of.

Thus the same force acting on a smaller area exerts a larger pressure. That is why it is easier to cut with a sharp knife than with a blunt one. Since the area of the edge of a sharp blade is extremely small, even a small force will exert a very high pressure.


Let us now calculate the pressure exerted by a liquid at the bottom of a container. Take equal masses, say 5 kg, of water in two rectangular containers as shown in figure. The force exerted by a mass of 5 kg of water is 5 kg x 9.8 ms-2=49 N.
In the first case water exerts this force over an area = 20 cm x 5 cm = 100 cm2=0.01 m2.

Hence the pressure exerted by water on the base of the container
=4900 Nm-2.

In the second case the same force now acts over an area=10cm  × 5cm = 50cm2 = 0.005m2.

Hence the pressure exerted in this case is
=9800 Nm-2.

Thus water exerts the same force but not the same pressure at the bottom of the container in these two cases.

We have seen that, like solids liquids also exert pressure. But unlike solids, liquids exert pressure not only at the bottom but also on the sides of the vessel in which they are contained. If you make a hole in the vessel, the liquid begins to come out of it. If your place your finger on the hole, you can feel the pressure.

Thus Pressure is defined mathematically as,

Thus when the area of contact is less, the pressure is more.

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