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

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