Re: "Weight vs. Mass"?


N:dlzc D:aol T:com (dlzc) wrote:
Dear Danny Dot:

"Danny Dot" <don't.mail.me@xxxxxx> wrote in message
news:1CrGg.21909$ph.20785@xxxxxxxxxxxxxxxxxxxxxxx
I just read a post that the person thought the word
"weight" in the English language is really used for
the mass. I wasn't sure at first, but now I agree.

It is not. Weight is force. Mass requires acceleration to
achieve weight.

It is true however that the pound (Imperial unit of force) has
been made proportional to the kilogram (SI unit of mass).

Mass on the Moon will be compared via a scale to other known
masses. Likely a 2000 # jack on the Moon will be much smaller
than an equivalent unit sold on Earth. And much more expensive.

David A. Smith

Oh, good grief!

Whenever anybody talks about the "net weight" of anything (what Danny
Dot was talking about), a term in the jargon of commerce and not in the
jargon of physics, that "weight" is ALWAYS mass, never force.

Furthermore, pounds are primarily units of mass, not units of force.
They are, by definition worldwide since 1959, exactly equal to
0.45359237. That is the definition legally required for the sale of
those apples Danny Dot was talking about, in any place where it is
legal to sell apples in pounds rather than kilograms.

Furthermore, those pounds do have a spin-off known as a pound-force,
just as there are kilograms-force spun off from kilograms. The only
diffence is that kilograms-force are not a part of the modern metric
system, the International System of units. Since the English units are
no longer fully supported and updated, nobody will ever tell us to stop
using pounds-force, without bother telling us to stop using pounds as
units of mass as well.

Kilograms-force are by definition 9.80665 newtons, exactly. But since
pounds-force do not have an official, universal definition, it isn't
even necessary that pounds-force bear the same relationship to
kilograms-force as pounds bear to kilograms. They often do, but a
different acceleration can be and sometimes is used to define
pounds-force.

Pounds-force were never well-defined units before the 20th century, and
even today they don't have an official definition.

Gene Nygaard

.

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