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.

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

Gift Giving Made Easy
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give the gifts they want
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On Mon, 19 Jun 2006 13:03:02 -0500, Chris W wrote:

the doctor tells you that you need to loose weight, that's easy just
move to the moon, done.

Hi Chris,

Language is a strange thing. For instance the doctor would probably
prescribe a laxative if he wanted you to loose weight.

73's
Richard Clark, KB7QHC

"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

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:
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

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

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: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
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

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

On Mon, 19 Jun 2006 12:01:37 -0700, Roy Lewallen
wrote:


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, you overlooked that the US, an earlier signup to SI, fixed the
spelling of metre in the US variant of SI.

Owen
--

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.

If you "weigh" 200 lb (no s at the end of unit symbols) on Earth, that
_is_ your mass, in normal usage in either the medical sciences or in
sports, which are of course the purposes for which we normally weigh
ourselves.

You can, of course, use those 200 pounds in the F = ma formula. For
example, if you accelerate those 200 pounds at 40 ft/s², the force is
8000 lb·ft/s², which is, of course, 8000 poundals, since a poundal is
the force which will accelerate a mass of one pound at a rate of 1
ft/s².


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.

Not at all. It indicates that you exert a force due to gravity of 1050
poundals (not lbs) on the moon. On Earth, you would exert a force of
somewhere in the neighborhood of 6410 poundals to 6450 poundals,
depending on your specific location.

Not only is it just as easy to use an unfamiliar unit for force as it
is to use some strange unit for mass, but the absolute
foot-pound-second system (which includes poundals) has been around
considerably longer than the gravitational foot-pound-second system
(which includes slugs), and rather than either of those systems, those
still using English units are more likely to use the engineering system
which includes both pounds and pounds force, but neither slugs nor
poundals. Since that system is not a completely coherent system, of
course, many of the formulas need to be adjusted with a g_c factor, a
dimensionless number equal to the ratio of the acceleration used to
define a pound-force to that used to define a poundal, or g_c = (32.174
ft/s²)/(1 ft/s²) = 32.174

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.

The symbol for kilograms is kg, not KG. There is nothing different
about the weight in the English units world, where the pound used for
this purposes is, by definition, exactly 0.45359237 kg.

Gene Nygaard