Where η is a constant known as the coefficient of viscosity and dv/dx is called velocity gradient.

The force of friction f between two layers each having area A sq cm, separated by a distance dx cm, and having a velocity difference of dv cm/sec, is given by: Viscosity is the force of friction which one part of the liquid offers to another part of the liquid. In other words, every layer offers some resistance or friction to the layer immediately below it. As we move from the centre towards the walls, the velocity of the layers keeps on decreasing. It is a type of flow with a regular gradation of velocity in going from one layer to the next. As we move from the wall towards the centre of the tube, the velocity of the cylindrical layers keeps on increasing till it is maximum at the centre. The layers which are in contact with the walls of the tube are almost stationary. Imagine the liquid to be made up of a large number of thin cylindrical coaxial layers. All parts of the liquids do not move through the tube with the same velocity. Hence, they are less viscous or have low viscosity.Ĭonsider a liquid flowing through a narrow tube. This is because of the weak intermolecular forces. The liquids which flow rapidly have a low internal resistance. Therefore, these liquids are more viscous and have high viscosity. This is because of the strong intermolecular forces. The liquids which flow slowly, have high internal resistance. It is the internal resistance to flow possessed by a liquid.

Viscosity units kgs