"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

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