Fluids at Rest describes key concepts about fluids including: 1) Density is defined as the mass per unit volume of a substance and can be expressed in units like kg/m3. 2) Specific gravity is the ratio of a substance's density to that of a standard substance, usually water. 3) Hydrostatic pressure is the normal force per unit area exerted by a fluid at rest, such as from water pressure increasing with depth.

1. Fluids at Rest

2. What is Fluid ?
Maybe liquid or gas, that is capable of flowing and t
hat changes its shape at a steady
rate when acted upon by a force tending to change its
shape.

3. DENSITY
 The MASS PER UNIT VOLUME of a substance.
(1)
𝜌 =
𝑚
𝑉
Units; kg/m3 , g/cm3 , slug/ft3

4.  weight-density or WEIGHT PER UNIT VOLUME:
(2)
𝐷 =
𝑊
𝑉

5.  Since 𝑊 = 𝑚𝑔, we have a simple relation between density
and weight-density:
(3)
𝐷 = 𝑝𝑔

6. SPECIFIC GRAVITY; RELATIVE
DENSITY
 Specific gravity or relative density of a substance is the RATIO OF ITS DENSITY TO THAT OF SOME
STANDARD SUBSTANCE.
(4)
𝜌 = density of the substance
𝜌w = density of water
𝜌r = relative density of substance
𝝆𝑟 =
𝝆
𝝆𝑤

7. HYDROSTATIC PRESSURE
 Pressure is the NORMAL FORCE PER UNIT AREA.
(5)
 Symbol for pressure is P
𝑃 =
𝐹
𝐴

8.  At Eq. (5) represents an average pressure over the area.
(6)
 In any small area where the change in A where there is normal force, the average
pressure is change in P = change in F / change in A:
𝑃 = lim
∆𝐴→0
∆𝐹
∆𝐴