Consider a cylinder is at rest. The fluid element of cylinder with the action of forces is shown in Fig.2.3

Let pdA be the force on section AB at a distance z from the datum.

dz be the height of cylinder

(p + ∂p/∂zdz)dA be the force on section CD whose distance from datum is (z + dz)

W be the weight of cylinder

Then,

Forces can be written as (in z-direction),

∑▒Fz = 0

pdA + W – (p + ∂p/∂zdz)dA = 0

or,

∂p/∂zdzdA = W

Or,

w = ∂p/∂z

ρ . g = dp/dz (ρ = Mass Density)

This suggests that rate of depth dp/dzis equal to the weight density of fluid.

The above equation can also written as-

p2 – p1= ρ. g (z2 – z1)

or,

p = . g .z

**Links of Previous Main Topic:-**

- Introduction about air standard cycles
- Properties of pure substances introduction
- Vapour compression refrigeration cycle introduction
- Basic fluid mechanics and properties of fluids introduction
- Fluid statics introduction
- Fluid pressure at a point
- Pascals law
- Pressure density height relationship

**Links of Next Mechanical Engineering Topics:-**