Since I rather mischievously made the original reference that instituted a
veritable blizzard of posting, and lacking the wisdom of David Bowman, I
must weigh-in on the subject. (somebody had to use the pun).
Please correct all mistaken interpretations:
Summary:
The discussion seems to be regarding the best definition of the term weight
as it relates to force acting on an object. I believe I have identified 3
or 4 positions that are probably self-consistant. I'm restricting my
discussion to a Newtonian viewpoint. (This doesn't mean the discussion has
no bearing on GR considerations; it simply means lets keep the discussion to
instances where GR corrections are unnecessary.)
1) Weight equals the force of gravity. For an object near the surface of the
earth this would have a magnitude of Mg down.
This is the predominate mode in introductory textbooks & I'm guessing a
quick vote of Phys-L would have more votes for this position.
If this is your viewpoint, it seems to me that there is no need to ever
mention the word weight in class. As we already have a perfectly good label
for the downward pointing force vector in the free-body diagram. Namely,
Mg, called "the force of gravity".
2) Weight is what a scale weighs when used in some reasonable manner. The
appropriate scale might be a spring balance, pan balance, equal arm balance
etc. Whatever is appropriate for the situation at hand. Allow gedanken
weighings as well.
This appears to be Leigh's, David's (Bowman) and John Mallincrodt's
viewpoint. It is also mine, but I bifurcate from them regarding bouyancy (I
think).
2a) Changes in bouyancy change the weight of an object. I religously
believe the scale, when I suspend the object in water compared to air I get
a different reading on my spring scale; so if weight is what the scale reads
then the weight must have change. I don't understand why the buoyancy
doesn't count people are willing to take the scale reading at face value if
you put the scale in a non-inertial reference frame, but not do so if you
immerse it in a fluid?
Please enlighten.
2b) Immersing object in fluid doesn't change weight, despite the scale
reading changing. (I don't understand this one yet.)
3) weight equals sum of all non-gravitational forces. I don't understand
this one either. Unless as modified by Bowman's old posting where he adds
the caveat of the scale providing all the non-gravitational forces. This
responds to my bouyance objection above. But I'd object to this on
operational grounds of "how do I weigh an immersed object?"; in particular
how do I do it with out comparing to a similar measurement in a vacuum?