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Mechanical Properties of Fluids

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Physics Part II

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Class 10 Class 12
Calculate the force required to take away a flat circular plate of radius 5cm and mass 2gm from the surface of water. The surface tension of water is 72 dyne/cm.

Radius of the circular plate, r = 5 cm 
Mass, m = 2 gm
Surface tension of water, T = 72 dyne/cm

 Weight of plate is, W = mg=2 x 980 = 1960 dyne

The circumference of circular plate is given by, =2πr=2π×5=31.4cm

The down pull on plate due to surface tension is given by, 
f=T=72×31.4=2260.8 dyne 

Therefore, total force required to take plate away from water surface is,



 =4220.8 dyne



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. 

What is fluid?

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


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.