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Elliotte Rusty Harold wrote:
> I think this confuses the units for mass and weight. For example, if
> I recall my high school physics correctly, kilogram is a unit of mass
> while pound is a unit of weight. If you fly to the moon your mass in
> kilograms stays the same but your weight decreases. In the metric
> system I think weight would be measured in dynes or newtons while in
> the English system mass would be measured in slugs.
Newtons and dynes are more often used as units of force,
not weight (weight is almost never an interesting quantity
in physics). Slugs are the canonical unit of mass in the
English system, but pounds can also be used as long as you
take care to distinguish pounds force and pounds mass (not
to mention pounds sterling :-)
> Of course in day-to-day usage among non-scientists on planet Earth,
> weight and mass are used interchangeably. Thus I wonder whether it's
> even possible to come up with a canonical list of all possible units,
> when not everyone agrees all the time what the untis are measuring.
It's not even a finite set. For any quantity of interest with
units "U", the time rate of change of that quantity is also
of interest and has units "U per second".
--Joe English
jenglish@flightlab.com
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