Abstract

In mechanics the principle of inertia is the result of an erroneous thought experiment (Gedankenexperiment) and not a law of experience. The inventor of the thought experiment was Descartes, Euler perfected it and postulated it as one of the fundamental laws of mechanics. Instead of describing the origin of the principle of inertia in great detail, I will try to capture the essence of the doctrine through the assumptions:

1. Only one body is moving in vacuo.
2. Velocity is a state of the body like its shape, its hardness or its colour. That is to say: velocity is absolute and belongs to the body and is therefore without any relationship to other bodies. In other words: velocity is an intrinsic property of the body.
3. The body is ?dead? and does not have inner forces in the meaning of motor or impetus.

and conclusions of the Gedankenexperiment:

1. The state of velocity v is persisting: v = const. Therefore, the motion continues in the present state of velocity, moving uniformly forward in a straight line. There is no sufficient reason for the body to change its state of velocity or its direction of movement.
2. Only an external force !F !can disturb the body persisting in its inert state (or in its laziness) !v = const., hence change this state. Without an accelerating or decelerating force !F,! the velocity and therefore also the momentum mv of the body is preserved: mv = const. (m... mass).
3. The quantity of the force necessary for a change of velocity (acceleration) is proportional to the mass m of the body, mass being understood as the quantity of matter.