Coefficient of viscosity of a liquid is defined as the viscous force acting per unit area between two adjacent layers of a liquid with a velocity gradient normal to the direction of flow of the liquid.

The formula for Coefficient of viscosity (η) of a liquid is given as,

where F= viscous force , A = Area , (dv/dx) = velocity gradient

η = F / A(dv/dx) .

SI unit of Coefficient of viscosity (η) of liquid is kg m^-1 s^-1.

Velocity Gradient is defined as the ratio of increase of velocity to the distance across which the increase occurs.

The formula for velocity gradient is given as,

Velocity Gradient = velocity/distance.

In fluid mechanics ,

Velocity Gradient is defined as the variation in velocity among the adjacent layers of the fluid. The difference in the flow of velocity between the  adjacent layers of the fluid is measured in velocity gradient.

The formula for velocity gradient is given as,

Velocity Gradient = Difference in flow velocity between the layers / distance between the layers.

SI unit of Velocity Gradient is s^-1.

Hydrostatic Pressure is the thrust (normal force) applied by a liquid at rest per unit area to the surface in contact with a liquid. Hydrostatic Pressure is also known as Pressure of liquid.

The formula for pressure is given as,

Pressure  =  Force /Area 

The pressure (P) exerted by liquids depends on Height (h) of the liquid column ,Density of liquid (ρ) and Acceleration due to gravity (g).

The formula for pressure of liquid can also be expressed as,

Pressure of liquid ( Hydrostatic Pressure) =  h ρ g.

Pressure is a scalar Quantity and SI unit of Hydrostatic Pressure is N / m² or Pascal (Pa)

The laws of kinetic friction are,

1. The force of kinetic friction always acts in the opposite direction to which body is moving.
2. The force of kinetic friction remains constant for moderate speed but it decreases with an increase in speed.
3. Magnitude of kinetic friction has a constant ratio to normal reaction between two surface. The ratio of kinetic friction is slightly less than that in case of limiting force of static friction.
4. The coefficient of kinetic friction is independent of the area and shape of the contacting surfaces.

The formula for law of kinetic friction is given as,

F_K = µ_k N ——— where F_K =Force of kinetic friction, µ_k= coefficient of kinetic friction and N=Normal force

The formula for coefficient of kinetic friction is given as,

µ_k = F_K / N

The laws of static friction are,

• When two surfaces are in contact , the frictional force is always tangential to contacting surfaces in a direction opposite to that in which the body tends to move.
• The limiting force of static friction is directly proportional to normal reaction between two surfaces in contact.
• The force of static friction depends on nature and materials of surface.
• The coefficient of static friction is independent of the area and shape of the contacting surfaces.

The formula for law of static friction is given as,

  F_S = µ_s N      ——— where F_S=Force of static friction, µ_s= coefficient of static friction and N=Normal force

The formula for coefficient of static friction is given as,

µ_s = F_S / N