Which of the following is correct about viscosity ?

Explanation:

Viscous force (F): When a layer of fluid slips or tends to slip on adjacent layers in contact, the two layers exert tangential force on each other which tries to destroy the relative motion between them.

• The property of a fluid due to which it opposes the relative motion between its different layers is called viscosity (or fluid friction or internal friction) and the force between the layers opposing the relative motion is called viscous force.
• It is caused by intermolecular forces and momentum transfer.
• It occurs in liquids, gases but not in solids.
• It is a non-conservative force, i.e. the work done will depend on the path along which the body was moving.

The force acting between the different layers of a fluid is given by:

\(F = - \eta A\frac{{dv}}{{dx}}\)

Where η = Coefficient of viscosity, A = Area of the plane and dv/dx = Velocity gradient.

A negative sign is employed because viscous force acts in a direction opposite to the flow of liquid.

Therefore, none of the first 3 options are correct.