Definition of Pressure, density, specific weight, and gravity

Pressure (P):

• If F be the normal force acting on a surface of area A in contact with liquid,then
pressure exerted by liquid on this surface is: P =F / A

• Units : N /m2 or Pascal (S.I.) and Dyne/cm2 (C.G.S.)

• Dimensions :- 

• Atmospheric pressure:- Its value on the surface of the earth at sea level is nearly
1.01*10 5N/ m2 or Pascal in S.I. other practical units of pressure are atmosphere,
bar and torr (mm of Hg)

• 1atm = 1.01 *10 5Pa = 1.01bar = 760 torr

• Fluid Pressure at aPoint:-  dP= dF/dA

Density ( ρ):

• In a fluid, at a point, density ρ is defined as: Mass/volume

• In case of homogenous isotropic substance, it has no directional properties, so is a scalar.

• It has dimensions [ML-3] and S.I. unit kg/m3 while C.G.S. unit g/ccwith
1g / cc = 10 3kg / m 3

• Density of body = Density of substance

• Relative density or specific gravity which is defined as :-

        

Specific Weight ( w):-

• It is defined as the weight per unit volume.

• Specific weight =Weight
                                 Volume

Specific Gravity or Relative Density(s):-

• It is the ratio of specific weight of fluid to the specific weight of a standard fluid.

Standard fluid is water in case of liquid and Hidrogen gas or air in case of gas.

Specific gravity. =.   Density 𝑜𝑓 𝐹𝑙𝑢𝑖𝑑
                             Density 𝑜𝑓 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝐹𝑙𝑢𝑖𝑑