"Peter Duniho" wrote in message ...
"David CL Francis" wrote in message
...
I see now at last what you are talking about, but to call the force of
gravity 'weight' seems curious to me.

What else would you call it? And perhaps more interestingly, what is
"weight" if not the force of gravity? I'm curious: if "weight" is not the
force of gravity, what is it? How would you describe "weight" without
noting that it's a force resulting from gravity?

Weight cannot be detected except
when a body is not in free fall.

Of course it can. Simply note your acceleration, multiply by your mass, and
you get your weight. The acceleration itself is proof of the force called
"weight". Without a force, there is no acceleration. In free fall, the
*only* force acting on the object is weight.

So in orbit an object has no weight.

Again, yes it does.

Gravity enables 'weight' on objects that are on the surface of a body to
be measured. Astronauts cannot weigh things in orbit.

Of course they can. As I described above, all they need to do is note their
acceleration, multiply by their mass, and they get their weight.

Just because you do not have a handy fixed object against which to measure a
force, that does not mean that the force does not exist. It simply means
that you need something other than a spring attached to a fixed object to
measure the force.

By your definition, if I were to jump from the roof of a two-story building,
for the period of time before I struck the ground, I would be "weightless".
Yet, by any reasonable definition of "weight", a "weightless" object would
not fall to the ground at all.

Pete

Weight is an ambiguous word--it has several different meanings.

Francis Weston Sears and Mark W. Zemansky, University Physics,
Addison-Wesley, 4th ed., 1970 used your definition, Pete, but said:

There is no general agreement among physicists as
to the precise definition of "weight." Some prefer to
use this term for a quantity we shall define later and
call the "apparent weight" or "relative weight." In
the absence of a generally accepted definition we shall
continue to use the term as defined above.

Most physicists today use David's definition, however.

Note that in Pete's definition the term "weightless" would never have
entered our language. Sure, there are many places where you would be
weightless in Pete's definition, some closer to the Earth than the
moon is, but we don't go there except in science fiction.

Of course, that only scratches the surface of the ambiguity in the
word weight. Consider all those labels at the supermarket and the
hardware sto "net weight" is not a physics term, and weight in
this context doesn't have either of the definitions you two are using.
That weight does not vary with the strength of the local
gravitational field--and it should not.

Same goes for "atomic weight" and "molecular weight" and "dry weight"
and "carat weight" and "troy weight" and "thrust-to-weight ratios" and
lots of other applications.

--
Gene Nygaard
http://ourworld.compuserve.com/homepages/Gene_Nygaard/
"It's not the things you don't know
what gets you into trouble.

"It's the things you do know
that just ain't so."
Will Rogers