Gravity that keeps us on earth; the weight of the air that lies upon us; centrifugal forces when we turn: Different mechanical forces act upon us each and every day. We are able to handle, use and master them on the basis of our experience and without any knowledge of physics. When it comes to describing them, however, one often has to leave behind the familiar in order to discover the rules. The fact that a light object falls just as quickly as a heavy one is contrary to our experience. The physical description does not take into account the effect of air resistance. And yet it can explain why an apple falls from a tree or why the planets circulate around the sun in elliptical orbits.