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
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give the gifts they want
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"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
--