Log Peridic 50m - 1300m
 

"Chris W" wrote in message
news:rFBlg.57799$9c6.28215@dukeread11...
Owen Duffy wrote:


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.


You were taught wrong. If you use pounds in a formula that wants mass
such as F=M*A you will get the wrong answer.

F = MA
F = 1 pound * 32 feet/s^2
F = 32 pound*feet/s^2

you will note that pound(mass)* feet/s^2 is NOT the pound force unit. the
pound force unit is slug*feet/s^2. there is nothing wrong with the pound
mass unit per se.

you alluded to slugs.

F = MA
F = 1 slug * 32 feet/s^2
F = 32 slugs*feet/s^2 = 32 pounds

anyone else having flashbacks to particle dynamics class?

Gravity

So lets say you weigh 200
lbs on earth where A = 32 ft/sec^2. You can then calculate your mass by
solving for M = F/A or 200/32 = 6.25.

When you are doing physical calculations it is very important to use the
correct units, other wise you calculations are meaningless. Suppose you
want to know what you will weigh on the moon where the acceleration due
to gravity is 5.25 ft/sec^2. F = M*A if you use 200 for your mass you
get, 200 * 5.25 = 1050, that indicates you would weigh 1050 lbs on the
moon. Which is clearly wrong. Trying again with the correct units and
you get, 6.25 * 5.25 = 32.8, now that sounds more like what you would
weight on the moon.

In the non scientific world, where the metric unit KG is used for
weight, M=F*A works just fine if you put what you call "weight" in KG in
for M in the formula.

It's arguable which method is better, using mass or force units for
weight. What you want to know is do you need to change your weight, if
the doctor tells you that you need to loose weight, that's easy just
move to the moon, done. What he really wants is for you to loose mass.
So your weight (force) can change with gravity, but your mass doesn't
change. Unless of course you loose weight ;)


--
Chris W
KE5GIX

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Log Peridic 50m - 1300m
 

Chris W wrote:
Owen Duffy wrote:


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.


You were taught wrong. If you use pounds in a formula that wants mass
such as F=M*A you will get the wrong answer. So lets say you weigh 200
lbs on earth where A = 32 ft/sec^2. You can then calculate your mass by
solving for M = F/A or 200/32 = 6.25. . .

That's 6.25 pounds mass, I presume, for someone weighing 200 pounds force.

In my entire engineering school curriculum, I had only two courses which
didn't use the metric system, Statics and Dynamics, taught by the civil
engineering department. I have vague recollections of pounds force,
pounds mass, slugs, and poundals. As often as not, my answers were off
by g^2, since I never could remember which ones already had
gravitational acceleration built in and which didn't. But I developed a
method to deal with it. When presented with a problem, I first converted
everything to SI units. Then I solved the problem and converted the
answer back to U.S. units.

What a horrible system! My hat's off to the Canadians, who had the will
to convert, and established -- and stuck with -- a systematic program to
do it. What the U.S. did was to declare the metric system to be official
("Mission Accomplished!") and change whiskey bottles from fifths to 750
ml (which was promoted by the booze industry because it made the bottles
just a little smaller and they could charge the same price). Wow.

Roy Lewallen, W7EL

Log Peridic 50m - 1300m
 

"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

Log Peridic 50m - 1300m
 

Roy Lewallen wrote:


Chris W wrote:

Owen Duffy wrote:


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.


You were taught wrong. If you use pounds in a formula that wants mass
such as F=M*A you will get the wrong answer. So lets say you weigh
200 lbs on earth where A = 32 ft/sec^2. You can then calculate your
mass by solving for M = F/A or 200/32 = 6.25. . .


That's 6.25 pounds mass, I presume, for someone weighing 200 pounds force.

No, it is 6.25 slugs of mass. There is no such thing as pounds of mass.
Sorry for leaving off the units in my last post. Just because
someone says x KG of force or x lbs of mass doesn't mean that KG can be
force and pounds can be mass.


Distance:
Meter, Feet
Force:
Newton, Pound
Mass:
KG, Slug
Time:
Second, Second (Can you imagine if there were different time units in
each system?)


All other units are derived from these. Actually Newtons and Pounds can
be derived from time, mass and distance. 1 newton = 1 KG*M/s^2 and 1
pound = 1 slug*ft/s^2. Which brings us right back to that fundamental
formula F = M*A, 200 lbs = 6.25 slugs * 32 ft/sec^2.


--
Chris W
KE5GIX

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Log Peridic 50m - 1300m
 

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
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give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
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Log Peridic 50m - 1300m
 

Thanks to all that have answered my original post - but the information that
I seek has not been answered.

I understand that section 207 of the FCC telecomm act 1996 contains
information that I might be able to use to help me in this matter. Where
can I obtain a copy of Section 207? I cannot find it posted on the FCC
website!

Thanks,



"Howard W3CQH" wrote in message
...
Looking for specs on any log periodic antenna that covers 50Mhz - 1300Mhz,
(Ham variety). Specs must also contain that it can withstand 80MPH wind
and 30 Lbs of ICE?

Thanks and best DXin.

de Howard W3CQH

Log Peridic 50m - 1300m
 

"Chris W" wrote in message
news:z8Dlg.57804$9c6.44111@dukeread11...
Roy Lewallen wrote:


Chris W wrote:

Owen Duffy wrote:


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.


You were taught wrong. If you use pounds in a formula that wants mass
such as F=M*A you will get the wrong answer. So lets say you weigh
200 lbs on earth where A = 32 ft/sec^2. You can then calculate your
mass by solving for M = F/A or 200/32 = 6.25. . .


That's 6.25 pounds mass, I presume, for someone weighing 200 pounds
force.

No, it is 6.25 slugs of mass. There is no such thing as pounds of mass.

please read Wikipedia. Owen is correct. pounds are firstly a unit of mass,
and secondly a unit of force. Wikipedia cites several sources.

200 pounds of mass weighs approximately 200 pounds of force on the surface
of Earth. 1 slug is 32 pounds of force on the Earth.

pounds-mass is standardized to kilograms, which are in turn standardized to
an alloy bar or other methods.

Gravity

Sorry for leaving off the units in my last post. Just because
someone says x KG of force or x lbs of mass doesn't mean that KG can be
force and pounds can be mass.


Distance:
Meter, Feet
Force:
Newton, Pound
Mass:
KG, Slug
Time:
Second, Second (Can you imagine if there were different time units in
each system?)


All other units are derived from these. Actually Newtons and Pounds can
be derived from time, mass and distance. 1 newton = 1 KG*M/s^2 and 1
pound = 1 slug*ft/s^2. Which brings us right back to that fundamental
formula F = M*A, 200 lbs = 6.25 slugs * 32 ft/sec^2.


--
Chris W
KE5GIX

Gift Giving Made Easy
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everyone who is arguing that pounds is not a unit of mass, please see:

https://carnot.physics.buffalo.edu/a.../msg00062.html

this post cites a NIST publication, which is definitive for the USA. there
is no room for argument.

if you don't live in the USA, well a pound can be anything you wish it to
be.

Gravity

Log Peridic 50m - 1300m
 

"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

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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