The change in velocity from layer to layer gives rise to shear stress in the fluid.The fluid layers in between the plates slide parallel to one another on account of molecules having different momentum crossing the fluid layers.The fluid layer in contact with the upper plate will move with velocity u and the fluid layer in contact with the lower plate, which is at rest will not move and will have zero velocity. That unit does not prove to be convenient for many uses, but note that the viscosity of water at 20C is almost exactly 1 if expressed in centipoise or.Now let the upper plate be moved by a force F.Consider a fluid which is at rest between the solid surfaces, separated by small distance Δy, along the y-direction, and extended infinitely in the other direction.All fluids possess viscosity though to varying degree. The kinematic viscosity of an oil (in stokes) whose specific gravity is 0.95 and viscosity 0.011 poise, is Kinematic viscosity is the product of dynamic viscosity and the density of the liquid. Test runs with water were made also at 5°, 40°, 60°, 80°, and 95 ° 0 to obtain comparative data on the viscosity of water and also to ascerta,in whether evaporation losses would cause serious error. This property is on account of cohesion and interaction between its molecules, which causes resistance to tangential or shear stress. When a tube of smaller diameter is dipped in water, the water rises in the tube due to viscosity of water. The viscosity of water at 20° 0 was assigned the value 1.00502 centipoises.
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