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Dear All, Sept 21, 2012
The recent discussion about transmission of electricity, reading of HEAT
and reading some of Energy... brings to the forefront a question that has
nagged me for decades. Perhaps someone can clarify.
In the typical elementary physics text we are told that velocity is a
vector quantity, and to quote one text "velocity... may be defined as the
rate of change of position in a given direction". But "In cases where the
direction of motion does not require to be considered , the term speed is
employed to express the rate of travelling."
Based on the above I would think that the rate of movement of light
should be called speed, i.e. speed of light but I think it is always called
velocity.
For example, if light from the sun is reflected from two 45o mirrors then a
beam of light could be directed from the earth back to the sun and, the
velocity would then be minus 3 x 10^10 cm/sec. Or if variously scattered or
reflected then the velocity, relative to the initial sun to earth direction,
would always be less than 3 X 10^10 and after several reflections might be
zero.
Is there some good reason why the speed of light is termed velocity of
light ? Or is it called velocity because the textbook authors forget what
they said in chapter one by the time they write chaper eleven ? (And they
all copy from each other)
Yt, Dave Webster, Kentville
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