A solid body moving through a fluid has a force pushing on it which depends on the type of fluid. You might imagine that it would be much harder to swim in honey than it is in water. The parameter which describes this difference is the viscosity (μ). The drag force also depends upon other parameters such as the surface area of the body and the fluid density, as you will discover in this chapter. The computer will be programmed to measure the speed of a sphere falling through glycerine and to calculate the viscosity. The measurements are made with photosensors and using machine language programming.

Force required to move a solid body through a fluid

In this section the physics of a sphere moving in a fluid will be discussed. There are two distinct regimes; if the sphere is moving slowly, the dominant force resisting its motion is due to viscosity. For rapid movement, the inertial resistance of the fluid due to its density is the dominant factor. The magnitude of the resistance and the functional dependence on sphere size, velocity, fluid density and viscosity can be estimated in a rough way for both cases. This gives insight into how the drag force behaves without getting lost in the mathematics. Indeed, with turbulent phenomena exact computations have not been possible.

Viscous resistance of a fluid arises from shear in the velocity profile of flow. If two flat plates have fluid between them, as shown in Figure 7.1, a force is required to move the top one at a constant speed in relation to the bottom one.