Viscosity –viscosity for gas

Viscosity for gas

If a gas flow in a tube it is seen that the velocity of the layer just touched in the tube is zero.It increase to the center.the velocity of center is maximum. So there must exist a velocity gradient.
It is due to the continuously movement of gas molecules. There is an interchange the molecules between to layer .as a result a part of momentum of one layer is pass over to another layer.so there happen a net decrease in relative velocity of a layer compare to another layer.So there must a internet friction whis is the viscosity of gas.To maintain uniform velocity gradient a force is applied to the direction of flow,which is the measurement of viscosity for gas.