"daestrom" wrote in message
...
"krw" wrote in message
t...
In article ,
says...
In article ,
krw wrote:
In article 39,
says...
NameNotImportant wrote in
m:
lbm?
I'm not sure on your units.
pounds (mass), lbm, as opposed to pounds (force), lbf, or lb.
It is necessary to distinguish between mass and force but they are
both
measured in pounds in the english system.
The "English" system uses the "stone" as the measurement of mass.
The pound ('lb') is the unit of *FORCE*.
The 'Stone' is a unit of mass, not "The unit of mass"
It is *the* unit of mass. The pound-mass is a recent abortion.
If you call the past 100 years or so, 'recent'. I myself have text-books
from the '50's that use this 'recent abortion' as you call it.
Considering the separation of force and mass was first worked out *after*
the original 'pound' for weight was in common use, it was necessary to
separate which 'kind' of 'pound' was being talked about. The one that
represents how much *force* is being applied to something, or the one that
describes how much resistance to acceleration something has.
But for a long time a 'pound' of something was a certain amount of
mass -or- the force applied to a surface by placing that certain amount of
mass on it (such as used in 'dead-weight' testers for pressure
instruments).
In a few obscure bits of engineering, you can even find the term
'kilograms of force' used. Obviously that is the force applied by placing
a kilogram of mass on top of something. You can even find some pressure
gauges calibrated to read 'kg/cm^2'. Proof that you can mess up things
even with the metric system. ;-)
I'm not sure how old the 'stone' is, but I suspect it too was around
before we knew the difference between force and mass. Stone is common in
UK still, but it never caught on in the colonies, even as far back as
colonial days when 'hundredweight' and 'long ton' were in common usage.
Trouble with pound-mass (lbm) and pound-force (lbf) is that to make F=MA
work out, you need to keep another 'conversion factor', the dreaded
g-sub-c (g-sub-c = 32.2 lbm-ft / lbf-s^2), around and figure out when to
throw that into the mix.
daestrom
In the early '50's there were two other units around- the poundal (1/g
pounds force) or a mass called a slug (g pounds mass). Learning mechanics
with these units (don't use them together)is worse than working in the
stone, furlong, fortnight set of units.
The poundal was introduced in 1879 as part of the "english set of units"
(Wikipedia is sometimes useful).
--
Don Kelly
remove the X to answer
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