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The purpose of mechanics is to describe how bodies change their

position in space with "time." I should load my conscience with

grave sins against the sacred spirit of lucidity were I to

formulate the aims of mechanics in this way, without serious

reflection and detailed explanations. Let us proceed to disclose

these sins.

It is not clear what is to be understood here by "position" and

"space." I stand at the window of a railway carriage which is

travelling uniformly, and drop a stone on the embankment, without

throwing it. Then, disregarding the influence of the air

resistance, I see the stone descend in a straight line. A

pedestrian who observes the misdeed from the footpath notices

that the stone falls to earth in a parabolic curve. I now ask: Do

the "positions" traversed by the stone lie "in reality" on a

straight line or on a parabola? Moreover, what is meant here by

motion "in space"? From the considerations of the previous

section the answer is self-evident.