Particle Motion in a Viscous Medium
In this chapter, we will consider the motion of aerosol particles through a medium that is composed of gas molecules, which offer resistance to the movement of the particles. The driving force of the motion is usually some external force such as gravity, and this is opposed by the resistance of the medium. The motion may be steady and rectilinear (i.e., in a straight line), or accelerated and along a curved path.
Using some simple arguments and assumptions, Newton developed an expression for the drag force on an object (such as an aerosol particle) moving through air. Consider a particle moving through a medium composed of molecules such that the particle is much larger than the molecular dimensions, as in Figure 10.1.