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Mechanical Properties of Fluids

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Physics Part II

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Class 10 Class 12
A 10x1.5x0.2 cm3 glass plate weighs 8.6 gm in air. Now it is immersed half in water with longest side vertical. What will be its apparent weight? (surface tension of water is 70 dyne /cm.)

The different forces acting on the plate are:

(i) Weight W vertically downward,
(ii) Upward thrust U vertically upward,
(iii) Surface tension force T in downward direction .

These forces are given by, 

Weight, W=8.6 x 980 = 8428 dyne

Upward Thrust, U=weight of water displaced 

                          =Volume of water displaced x density of water x g
                           =12(10x1.5x0.2)x1 x 980=1470 dyne


T = Total length of the water in touch with plate x surface tension

   =2(1.5 + 0.2) x 70 = 238  dyne

Now the apparent weight, 

W+ W + T - U = 8428 + 238 - 1470

                         = 7196 dyne

                         = 7.434 gf


What is fluid?

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


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 hydrodynamics?

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

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.

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.