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Mechanical Properties of Fluids

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Physics Part II

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Class 10 Class 12
What is atmosphere and atmospheric pressure? Discuss the Torricelli’s experiment to measure the atmospheric pressure.

Atmosphere is a gaseous envelope surrounding the earth and the pressure exerted by the atmosphere is called atmospheric pressure.

The cause of atmospheric pressure is motion of air molecules which are in continuous mtion.

The molecules strike the surface of body placed in it and exert a huge force.

To measure the atmospheric pressure,

i) Torricelli took a meter long graduated tube and filled it with clean and dry mercury.

ii) By closing the tube with thumb, he inverted the tube in a cistern ( a tub filled with mercury) as shown in the figure.

iii) He observed that the level of mercury first fell down and finally stayed with a column of height 76cm in the tube above the free surface of mercury in the cistern leaving behind vacuum.

iv) At point C, vacuum is formed. Therefore, pressure at this point is zero. Point B in the tube is 76cm below C. So, pressure at B is given by, 

PB =PC + ρgh = 0 + ρgh =  ρgh 

where ρ is density of mercury and h be the height column of mercury in the tube above point B.

In the given figure, point A is at the interface of air and mercury, therefore it is both in air and as well as in mercury.

Thus pressure at point A is equal to atmospheric pressure, i.e.

PA = Atmospheric pressure as vertical height between A and B is zero, therefore pressure at A and B is same, i.e.


Atmospheric pressure = pgh
                                 = 13600 x 9.8 x 0.76
                                 = 1.013 x 10N/m


What is hydrodynamics?

Hydrodynamics is the branch of science that studies about the force exerted by the fluids or acting on the fluids. 

Do intermolecular or inter-atomic forces follow inverse square law?

No. Intermolecular and inter-atomic forces do not obey the inverse square law. 

What is hydrostatics?

Hydrostatics is the branch of fluid mechanics that studies incompressible fluids at rest. The study of fluids at rest or objects placed at rest in fluids is hydrostatics.

What is fluid?

Any material that can flow is a fluid. Liquids and gases are examples of fluid.


Why solids have definite shape while liquids do not have definite shape?

Solids: Intermolecular forces are very strong and thermal agitations are not sufficiently strong to separate the molecules from their mean position. Solids are rigid and hence they have definite shapes.
Liquids: In liquids intermolecular forces are not sufficiently strong to hold the molecules at definite sites, as a result they move freely within the bulk of liquid, therefore, do not possess definite shapes. Liquids take the same shape as that of the container.