Fluid Statics

Matter exists in three states solid, liquid and gas. Any state of matter that can flow is a fluid. Liquids and gases are therefore, referred to as fluids.

Density :

Density of a substance is the mass per unit volume. Homogenous bodies have uniform density:
ρ = M/V

The unit of density is kg/m3 in S.I. system. Dimension of density is given by [ρ] = = [M] [L-3].

Relative Density or Specific Gravity :

Relative density of a substance is the ratio of the density of the substance to the density of pure water (ρw = 1000 kg/m3) at 4°C.

ρr = ρ/ρw.

Relative density is dimensionless

Illustration : A hollow metallic sphere has inner and outer radii respectively as 5 cm and 10 cm. If the mass of the sphere is 2.5 kg, find (a) density of the material (b) relative density of the material of the sphere.

Solution: The volume of the material of the sphere is

V = 4/3 π(r23 – r13)

= (4/3)x 3.14 x 0.000875 m3
= 0.00367 m3

(a) Therefore, density of the material of the sphere is
ρ= M/V = 2.5/0.00367 kg/m3

= 681.2 kg/m3

(b) ρr = 681.2/1000 = 0.6812

Density of a mixture

Let a number of substances of masses M1, M2, M3 etc., and densities ρ1, ρ2, ρ3, etc. respectively be mixed together. The total mass of the mixture is M1 + M2 + M3 + ….

The total volume is M1/ ρ1 + M22 + M33 + …. , provided that the substances retain their individual states within the mixture.

Therefore, the density of the mixture is

The same expression may written in terms of the volumes:

The density of the mixture is

where V1, V2, V3 … represent the volumes of substances of densities ρ1, ρ2, ρ3 . . . in the mixture

Illustration : Two liquids of densities 2.5 gm/cm3 and 0.8 gm/cm3 are taken in the ratio of their masses as 2:3 respectively. Find the average density of the liquid combination.

Solution: Let the masses be ‘ 2 m ‘ gm and ‘ 3 m ‘ gm respectively. Therefore, the volume of the first liquid of density 2.5 gm/cm3 is V1 = 2m/2.5 cm3

That of the second liquid is V2 = 3m/0.8 cm3

Total volume V = V1 + V2 = 2m/2.5 + 3m/0.8 cm3

Total mass = 2m + 3m = 5 m gm

Therefore, the average density


= 10/9.1

= 1.09 gm/cm3

Exercise : Two miscible liquids of densities 1.2 gm/cc and 1.4 gm/cc are mixed with a proportion ratio of their volumes equal to 3:5. What is the density of resulting liquid?

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