PART - 2 CH - 9 Mechanical Properties of Fluids — Physics STD 11 Science — Question

There is a velocity gradient in a flowing fluid due to which the upper layer flows at a higher velocity than the lower layer. Thus there is relative lag in successive layers.
Just as the force of friction acts between two surface moving with relative velocity, in the same way a type of friction occurs due to relative velocity in successive layers of liquid. For this reason, the slower moving layer creates obstructions on the faster moving layer.
Energy is required to overcome this obstruction and maintain the flow.
The force creating obstruction are called viscous forces and this property is called viscosity.