and ..............................(1)

The negative sign implies that the direction of force is opposite to velocity.
            .............................................(2)
where, the proportional constant is called coefficient of viscosity. The cgs unit of is called 'poise' in honour of the French scientist Poiseulli.

The viscous force can be experienced by performing simple experiment. Take two parrallel glass plates. These plates are seperated by a layer of water. It would be relatively easier to slide the top plate with respect to the bottom plate. Instead of water if we use castor oil, it would be harder to slide the plate. This is because of larger coefficient of viscosity, in castor oil. The values for water and castor oil are respectively 1.005 and 9.86 poise.

The equation (2) can be written as follows by replacing dv and dz as v and z respectively,
            .............................................(3)