Mechanics of Fluids and Pressure - Part I

Liquids and gases are called fluids because they flow and are able to take any shape. Since constituents of fluid, such as atoms or molecules, are not held tightly by their neighbours (opposite to that in the case of solids), fluids have to be held in containers. Liquids take up the shape of the container, where as gases expand to occupy the entire volume of the container.  The molecules of the fluid are constantly in motion and hence exert a force on their neighbouring surfaces. This force is measured in terms of per unit area and is called pressure. A system is stable only when pressure is equalized in all directions.

What we will study in this chapter :
1. Thrust and pressure
2. Pressure in liquids
3. Pascal’s law of transmission of pressure in fluids
4. Atmospheric pressure and its measurement
5. Measurement of density of unknown liquids
6. Boyle’s law and Charles’ law

1. Thrust and pressure  
Thrust is a total force in a particular direction. The unit of thrust, therefore is the same as that of force : Newtons (N). Pressure is the force or thrust applied per unit area.

                              Force (or thrust)
Pressure (P) =    
                                      Area

A unit of pressure is Newtons/m² or N/m².  The unit is also called a Pascal (Pa). There are other units in use also but these are the units in the M.K.S. system.

To appreciate the difference between thrust and pressure, do the following experiment. Take two trays filled with sand and one heavy rectangular block that measures 1 kg. Keep the first block on its side in one tray and the second block on its base in the second tray. You will notice that the first block has sunk deeper in the sand. Although the weights of the two blocks is the same then why does this happen?  Since the weights are same, the thrust or the force applied is the same in both the cases.  But the area on which this force is acting is different in the two cases. In the first case, the area is smaller hence the pressure is more. In the second case the area on which the force is acting is larger, hence the pressure is less. Thus, we can explain why the first block sinks into the sand more than the second block; in the first case the pressure applied is more.  

We can see several application of pressure in our everyday lives : cutting tool edges are sharper than their heads, a pressure applied on the head is magnified at the edge. The same is true with pin heads or nails, they can be pushed with minimum pressure on the heads.

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