The reader has probably noticed that while illustrating the law of conservation of mechanical energy we persistently repeat: “in the absence of friction, if there were no friction …” But friction inevitably accompanies any motion. What is the significance of a law which doesn’t take into account such an important practical circumstance? We shall put off answering this question and consider now some consequences of friction.

Frictional forces are directed against motion, and so perform negative work. This causes an unavoidable loss of mechanical energy.

Will this inevitable loss of mechanical energy lead to a cessation of the motion? It is not difficult to convince oneself that not every motion can be stopped by friction.

Imagine a closed system consisting of several interacting bodies. The law of conservation of momentum is valid, as we know, in relation to such a closed system. A closed system cannot change its momentum, so it moves rectilinearly and uniformly. Friction within such a system can change relative motions of parts of the system, but cannot affect the speed and direction of the motion of the entire system as a whole.

There exists still another law of nature, called the law of conservation of angular momentum (we shall make its acquaintance later), which does not permit friction to destroy the uniform rotation of an entire closed system. Therefore, the presence of friction leads to the cessation of all motions within a closed system of bodies, not obstructing only the uniform rectilinear and the uniform rotational motion of this system as a whole.

If the Earth does slightly change the speed of its rotation, the cause of this is not the friction exerted by terrestrial bodies against one another, but the fact that the Earth is not an isolated system.

As for the motions of bodies on the Earth, they are all subject to friction and lose their mechanical energy. Therefore, such motion will always cease if not supported from without.

This is a law of nature. But if one succeeded in tricking nature? Then … then one might be able to bring about perpetuum mobile, which is Latin for “perpetual motion”.