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Re: Introducing the United (fps) System
Posted:
Apr 23, 2000 8:42 PM
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In a previous article, "theresa knott" <theresa@knott16.freeserve.co.uk>
writes:
>
><gandg@snet.net> wrote in message news://8dvrta$rc3$1@news.netmar.com...
Snip<
>
>>Its a temptation to call it
>> 10, so that it fits in better with the base 10 SI system. Have I got it
>yet?
>
>No the reason that is called 10 is just because 10 is a nice round number
>that is easy to remember and calculate with. It has *nothing* to do with
>fitting it into SI. It's just the same with other constants, like for
>example the speed of light. It is usually quoted as being 3 * 10^8 m/s but
>it is not exactly that number. its just that round numbers are easier to
>calculate with.
>
Awe come on Theresa: Level with me: SI is supposed to be exact; thats the
claim for why we are labeling packages with metric units. So we won't get
ripped off by crooks who would label a package put up at a high location,
like maybe packaging drugs high up in the Andes, where the strength of
gravity and the free fall acceleration is less, so that the product weighs
less, and then they sell it at some lower elevation where the strength of
gravity and the free fall acceleration (g) is greater and turn a huge
illegitimate profit.
>g is a *measured* quantity. In SI the units are powers of 10. Units are
>defined they are not measured. Do you see the difference ?
I sure do: g varies at various locations on Earth depending on the elevation
and the latitude, where Earth's centrifugal effect increases toward the
equator. g is whatever it is: It _averages_ about 9.806 65 m/sec^2 at Earth's
surface: SI defines that as "standard" gravity and the weight of the kilogram
on Earth is 9.806 65 newtons. It's wishful thinking to say SI units are
powers of ten: On the moon and Mars a kilogram will weigh considerably less!
If we were
>suddenly transported to say Mars then g would change because the surface
>gravity of Mars is different from here,
Right.
but the SI system of units would
>remain the same because units are defined not measured.
A kilogram of matter would still be the same amount of matter if it was
sealed in a container, but its net weight would change in proportion to g! A
newton would still be a unit of force, but the net weight of a kilogram would
be fewer of them. There's no way to define weight, or free fall acceleration
as a power of ten: 10 _is_ a nice round number that is easy to remember and
calculate with; but g will not cooperate: It is what it is depending on where
it's at:
Gramp, Shead a'course.
>
>Theresa Knott
>
>
>
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