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 oppose 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.
The force acting between the different layers of a fluid is given by:
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.
EXPLANATION:
Viscous Force is defined as the internal frictional force of fluids.
The Viscosity is the resistance that a system offers to flow when it is subjected to a shear stress.
Gravitational Force is defined as the force which attracts any two objects that posses mass.
Magnetic Force is generated between two electrically charged particles in motion.
Frictional Force is defined as the force that opposes the motion of an object.