Jeroen said:
-On [20080212 22:15] said:
Note that Google will give a calculator result for "1 kilogram in
pounds", but not for "1 kilogram in inches". I wonder why not? After
all, both are conversions of incompatible measurements, ie, they
measure different things.
Eh? Last I checked both pound and kilogram are units of mass, so where is
the incompatibility?
He's saying something that's conditionally true depending on the system
of units you're using, hence your (quite understandable) confusion.
Once upon a time there were no physicists. In this happy-go-lucky era,
certain people living in a certain area of the world had a unit of
measurement for how hard gravity pushed something into the ground, and
how hard it was to push something along the ground. The figure was
called "weight," and in the particular area we're talking about, the
unit associated with it was called the _pound_.
Then physicists came along and pointed out that those two things aren't
quite the same thing, though no one had really noticed it before. If
you lived on a lower-gravity world, for instance, like the Moon or Mars,
then it would be easier to lift something, but it would still resist
being pushed just as much. If you were floating in space, far away from
any gravitating bodies, then that something wouldn't be being pushed
into any ground at all, but still would have just the same resistance to
being pushed (these bastard "somethings" don't like being shoved around,
you see).
The names of those two notions ended up being called "weight" (or
"force") and "mass." But what to do about the lowly pound? It's kind
of both, as we already discussed, but in proper physics it can't be. So
you have to split it into two units -- one for mass, one for weight.
For brand new metric systems (in all their variants), their units were
made up from scratch and so this didn't present a problem. So how did
they do it?
The answer is that different subgroups of those who used the pound did
it differently. Some accepted pound as the unit of mass, and invented a
unit of weight, called the _poundal_. Some took the pound as being a
unit of weight, and invented the _slug_ as the corresponding mass unit.
Some went so far as to effectively invent two new units: the
_pound-mass_ and the _pound-force_, and because of their names I don't
have to tell you which is which.
So there's a hodge podge of different rationalized unit systems for
dealing with the pound and its brethren, and different people are taught
different things and are perpetually confused. And not much good comes
of it.
And the rest of us just use SI. (And if you bring up the
_kilogram-force_, I'll just cry.)
