ABSTRACT
This chapter aims to instruct the reader in the art of obtaining approximate solutions from dimensional estimates with the aid of a simplified model of the phenomenon to be investigated. It considers the limits of small and large energy and traces out how the corresponding expressions match up in the intermediate-energy region. The chapter deals with the limiting case of small velocities; then the resistive force determined by the viscosity of the medium. It also considers the problem of finding the resistive force when a body moves in a viscous medium with arbitrary velocity. The parameter which defines the notion of "small" as opposed to "large" velocities may be found by forming a quantity with the dimensions of velocity from the viscosity, the density of the medium and the dimensions of the body. "Model" approximations mean those derived from limiting cases or simplified variants of the problem in question.
