"Owen Duffy" wrote in message
...
On Sun, 18 Jun 2006 07:10:56 -0500, "gravity"
wrote:


Given the looseness of use of the unit lb to specify mass and
(incorrectly) force, it is a bit ambiguous... but he probably means
mass.

lbs is always force as far as i know. slugs is mass.

so long as we are confined to the planet earth, there is no difference
really. 1 kg (mass) always weighs 2.2 lbs (force). obviously if you go
to
the moon ...

kilogram, slug -- mass
newton, pound -- force


I was taught (in imperial units) to differentiate mass (pound) and
force (pound-force). That learning stood me well when we changed to SI
(metric) part way through school.

Practice may be different in different places, but I suspect that it
is laxness on the part of practitioners who refer to force in units of
pounds.

I just had a look at Wikipedia (which isn't the oracle), here is their
summary:

"The pound is the name of a number of units of mass, all in the range
of 300 to 600 grams. Most commonly, it refers to the avoirdupois pound
(exactly 453.59237 g), divided into 16 avoirdupois ounces. There is
also a unit of force corresponding to the avoirdupois pound, see
pound-force."

Wikpedia highlights just another aspect of the unit, its flexibility!

Owen
PS: a slug is a unit of mass, and equivalent to about 14.6Kg or
32.2lbs. I don't think it is in wide use!
--

i first heard of slugs at age 7, but i've never seen them used in an
engineering class.

we used SI almost exclusively in university and high school. i was taught
there that pounds is a unit of force (not mass). however Wikipedia claims
pounds is a standardized unit of mass, not force.

so we are both right really.

as i've noted in another post, 1 slug at in Earth's gravitational field is ~
32 pounds, so it's a convenient unit to use.

so basically if NIST (or whoever) defines it as mass, then we are stuck with
it.

"honey do i look fat in this dress?"
"no baby, you are no more than 5 slugs or so."

Gravity

gravity wrote:


i first heard of slugs at age 7, but i've never seen them used in an
engineering class.

we used SI almost exclusively in university and high school. i was taught
there that pounds is a unit of force (not mass). however Wikipedia claims
pounds is a standardized unit of mass, not force.


I don't care who says pounds is a unit of mass, they are wrong! If you
use pounds in a formula that wants mass, your answer will be WRONG.


--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com

"Chris W" wrote in message
news:zcDlg.57805$9c6.18712@dukeread11...
gravity wrote:


i first heard of slugs at age 7, but i've never seen them used in an
engineering class.

we used SI almost exclusively in university and high school. i was
taught
there that pounds is a unit of force (not mass). however Wikipedia
claims
pounds is a standardized unit of mass, not force.


I don't care who says pounds is a unit of mass, they are wrong! If you
use pounds in a formula that wants mass, your answer will be WRONG.


please read the NIST publications which define the Avoirdupois pound in
terms of kilograms. this is not debateable since NIST is *the* authority in
the US.

Gravity

--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com

gravity wrote:

please read the NIST publications which define the Avoirdupois pound in
terms of kilograms. this is not debateable since NIST is *the* authority in
the US.

The NIST publications define the numeric conversion factors, not the
legality for use of any particular terminology.

The approximate conversion factor for a pound (mass) is 0.4535924 kilogram.

The approximate conversion for a pound (force) is 4.448222 newton.

The gravitational acceleration, small g, is defined as exactly 9.80665
in SI units, but it is not similarly defined in inch/pound units.

Interestingly enough, however, is that the ratio of pound-force per
pound (lbf/lb) (thrust to mass ratio) is exactly converted to newton per
kilogram (N/kg) by the factor 9.80665.

The position of the US Government can be summarized from the following
excerpt found in Federal Standard 376B, Preferred Metric Units for
General Use by the Federal Government.

In the intro to the section on mass there is a note that says,

*** There is ambiguity in the use of the term "weight" to mean either
force or mass. In general usage, the term "weight" nearly always means
mass and this is the meaning given the term in U.S. laws and
regulations. Where the term is so used, weight is expressed in kilograms
in SI. In many fields of science and technology the term "weight" is
defined as the force of gravity acting on an object, i.e., as the
product of the mass of the object and the local acceleration of gravity.
Where weight is so defined, it is expressed in newtons in SI. ***

The document then goes on to show many conversion factors from both
pounds (mass) and pounds (force) to SI units. No indication that one is
more legal or correct than the other.

Soooo, use pounds any way you wish; just do the math correctly.

73,
Gene
W4SZ

gravity wrote:


please read the NIST publications which define the Avoirdupois pound in
terms of kilograms. this is not debateable since NIST is *the* authority in
the US.

The NIST isn't the authority on the laws of physics. One of the most
basic of which is F=MA, if you use pounds for mass in that formula you
get the wrong answer. Nothing that NIST says will change that.

200 lbs = 6.25 slugs * 32 ft/sec^2. Using pounds, the formula won't
work any other way.

--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
http://thewishzone.com

Chris W wrote:
gravity wrote:


please read the NIST publications which define the Avoirdupois pound in
terms of kilograms. this is not debateable since NIST is *the* authority in
the US.

The NIST isn't the authority on the laws of physics. One of the most
basic of which is F=MA, if you use pounds for mass in that formula you
get the wrong answer. Nothing that NIST says will change that.

200 lbs = 6.25 slugs * 32 ft/sec^2. Using pounds, the formula won't
work any other way.

Bull****.

Here are a couple of other ways that work just fine with FFU:

200 lb * 32.2 ft/s² = 6440 pdl

0.52 slinch * 386 in/s² = 200 lbf

Be sure to distinguish pounds force (lbf) from pounds (lb), and units
of measure should remain unchanged in the plural, without adding any
language-specific "s" or whatever.

Gene Nygaard