Motion (source code)
= Motion
{wiki}
<Motion> is a change of <position> with <time>, described relative to a <reference frame>. A book resting on a train has constant <position> in the train's frame while its <position> changes in a frame attached to the track. Neither description is complete until the frame and the measured <time> have been specified.
A record of successive <positions> gives a <trajectory>. Comparing nearby measurements gives <velocity>, and changes of <velocity> give <acceleration>. A camera recording a rolling ball at known time intervals provides a simple measurement: unequal gaps between successive images reveal a changing speed. The description of this record is <kinematics>; explaining the change using <forces> belongs to <particle dynamics>.
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