We remarked on the obvious nature of the two kinds of energy possessed by a falling stone. What is not quite so obvious is the internal energy of the molecules inside the stone. The molecules of a substance are in continual motion. According to the state of the substance, they possess kinetic energy of motion or vibration together with potential energy resulting from the attractions and repulsion they exert on one another. Internal energy increases as the temperature of a body increases.
Scientists sometimes refer to the k.e. and p.e. of a body taken as a whole as being macroscopic (= visible to the naked eye). Internal energy may be described as microscopic (= invisible to the naked eye). However, it is possible to demonstrate the reality of molecular motion as we shall see when we discuss the Brownian motion (page 137).