Friction between solids was dealt with in chapter 2. We shall now say something about friction in fluids. Bodies moving through fluids, i.e., liquids or gases, experience a retarding force
otherwise known as fluid friction or viscous drag. This is a factor involved in the design of ships, aircraft, and. other vehicles. In order to achieve a design in which the energy-wasting effects of drag are reduced to a minimum, calculations are made and small-scale models are constructed which are then tested in water tanks and wind tunnels (Fig. 13.14). In the field of pure physics Sir George Stokes, an eminent mathematician and physicist of the nineteenth century, investigated the effects of viscous drag on small
spheres falling through liquids. Unlike bodies falling in a vacuum which accelerate constantly at 10m/s2, these spheres were found to acquire a steady terminal velocity. He found that they experienced an upward retarding force, F, which depended on their radius, their velocity, and a constant called the viscosity of the liquid. Besides this there are two other forces acting on the sphere namely, (see Fig: 13.15) (i) the force of gravity, W, acting downwards and (ii) the upthrust, U, of the liquid as given by Archimedes principle (page 128) which acts upwards. These two forces have a resultant (W-U) in the downward direction. At constant temperature, (W-U) is constant whilst the viscous drag, F, increases with velocity.
If, therefore, a small metal sphere is allowed to fall through the liquid it at first accelerates. As it does so its velocity increases, with consequent increase in the viscous drag. Eventually F becomes equal to (W-U) and the total resultant force on the sphere is zero. In accordance with Newton’s first law of motion the sphere then falls with a uniform velocity known as the terminal velocity.