we all know from basic idea that all liquid don't flow on same speed.such flow of water is greater than glycerol.there must be a internal friction which vary liquid to liquid.
for laminar flow of an liquid in a tube the velocity of layer just in touch with the side of tube is zero.but velocity increase as we proceeds towards center. the velocity of layer of the center is maximum.
thus there exist which is known as the velocity gradient between different layer of liquid.
This is happen for internuclear
attraction between molecules of two different layer.For inter nuclear attraction a molecule moving in a layer tend to impede the flow of different molecule of the adjacent faster layer.as a result the velocity of the faster layer decreases unless this decrease is prevented by applying the force towards forward direction. the velocity of faster moving layer going on decrease and ultimately it will become zero.at this state the liquid flowing stop.thud there is a tendency to decrease the relative motion as if there is a tangential force dragging backwards. this is viscus force. this is can be expressed in terms of tangential force which is needed to keep the speed of different layer constant.
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