Friction and viscosity are the two intermolecular forces that come into play whenever relative motion occurs. The difference between them is that friction retards the movement of the surface, and viscosity retards the flow of liquid between the different layers.

The Force Between the Surfaces

In case of friction, if a ball is allowed to move on the surface then it moves some distance and stops after a certain time. This is because no surface is completely smooth and it has non-uniformities in its layers.

Similarly, when honey is spilled out of a jar then it flows through a small distance but the internal resistance created between the different layers of the honey slows down the motion of the honey. This is also the same case in which if an iron-ball falls into water then different lavers of water restrict the motion of the iron ball.

Viscosity and the Fluids

The internal resistive force between the layers of a fluid that is in motion is called viscosity. This internal resistance is a property of all fluids and it is measured by the cgs units.