Since little is known concerning the laws of plastic flow, it is often convenient to use approximate methods of calculation based on the law of Poiseuille for simple viscous liquids. Neglecting the kinetic energy correction, the reciprocal of the viscosity isFluidity = 4qπR3PR2L = YX(1)whereq= rate of flow in cm3per secondP= pressure causing flowRandL= radius and length of capillary