The unit of viscosity is

1. Force:

(i)  Force: It is a push or pull, which changes or tends to change the state of rest or uniform motion of a body or changes its direction or shape.

(ii) A force can (a) move a stationary body, (b) stop a moving body, (c) slow down a moving body, (d) make a moving body move faster, (e) change the direction of a moving body, (f) change the shape of objects.

(iii) The SI unit of force is newton.

2. Thrust and Pressure:

(i) Thrust is the force applied on a surface perpendicular to it.

(ii) Pressure is defined as the thrust per unit area.

(iii) SI unit of pressure is pascal. It is the force of magnitude $1 \mathrm{N}$ acting on $1 {\mathrm{m}}^2$ area.

(iv) In case of solids the pressure increases with the increase in force or the decrease in the area of cross-section and vice-versa.

3. Force and Pressure Exerted by Liquids:

(i) Fluids are the substances which can flow, i.e., liquids and gases.

(ii) In case of liquids and gases (fluids), the pressure increases with the increase in height (depth) and density.

(iii) Liquids and gases exert same pressure in all directions at a given depth.

(iv) The pressure of air decreases with the increase in height from the ground level.

(v) Atmospheric pressure: The pressure exerted by the air on the bodies on the surface of earth. It is measured by a device called a barometer.   

(vi) $1$ atmospheric pressure$=1 \mathrm{atm}=$pressure due to $76 \mathrm{cm}$ of mercury in a barometer $=1.01\times {10}^5 {\mathrm{Nm}}^2$.

(vii) Pascal’s law states that the pressure applied at any point of a liquid at rest, in a closed system, will be distributed equally through all regions of liquid. Hydraulic lift, automobile brake system and hydraulic press are its applications.

4. Friction: 

(i) The force of friction always acts in the direction opposite to the direction of relative motion.

(ii) Static friction: It comes into play when we try to move an object at rest.

(iii) Limiting friction: The maximum force of static friction acting, when the surface of one body is about to slide over the surface of another body.

(iv) Sliding friction: The force of friction acting between the surfaces of two bodies sliding upon one another. It is smaller than static friction.

(v) When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than sliding friction.

(vi) Lubricant: A material which, when applied between two surfaces in contact, reduces the friction.

(vii) Ball-bearing: A device used for reducing rolling friction, by making an axle to move over metal balls placed around it radially.

5. Surface Tension: 

Surface tension is defined as the amount of force acting per unit length of the surface of a liquid. Its unit is $\mathrm{N} {\mathrm{m}}^{-1}$. 

6. Viscosity

The force which opposes the relative motion of successive layers of liquid when it tries to move is known as viscous force and the property responsible for this is viscosity. Its unit is poise in CGS system and $\mathrm{N} \mathrm{s} {\mathrm{m}}^{-2}$ in SI system