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Re: [Phys-l] definition of weight



On 11/01/2006 06:30 PM, Hugh Haskell wrote:

Defining weight as the gravitational force on an object gives the
impression that when you are "weightless" (a very common term, at
least superficially understood by most people),

Forsooth it *is* my impression that astronauts are weightless
relative to the frame attached to their space station. I do
not consider my understanding of this issue to be superficial.

that there is no
gravity acting on you, and of course that is not true,

Saying "of course" doesn't make it true, let alone self-evident.
In fact, what's true in your frame is not true in the space
station frame.

Far better, IMO, to define weight as what the bathroom scale reads
when you step on it, regardless of where you are or what
circumstances you find yourself in

That definition has several serious problems, as discussed at
http://www.av8n.com/physics/weight.htm#main-instrumental-def

* This is an “instrumental” definition. Like all instrumental
definitions, it is open to all sorts of questions about what happens
if the instrument is miscalibrated, or outright broken. There are serious
philosophical questions here, and also nontrivial practical questions when
we start asking about your weight on the moon, or your weight aboard a
space station, since scales that behaved fine on the earth might well
misbehave in other situations.

You might try to solve this class of problems by talking about an
“ideal” scale, but defining what you mean by that is no easier than
defining “weight” from scratch.

* If you were to take the scale reading literally, you would be in error,
because you would have neglected the buoyancy correction.

* Almost all scales are /intended/ to measure mass, not weight. For
example, consider kitchen scales. If the cookie recipe calls for 1/4
pound of butter, the /intention/ is presumably to use the same mass of
butter even if I am baking cookies on the moon.

* In practice it is not particularly uncommon to find two scales in
the same room, one of which (to a good approximation) responds to mass
while the other (to a good approximation) responds to weight.

Bottom line: Defining weight in terms of whatever “the” scale reads runs a
large (and quite unnecessary) risk of spreading misconceptions.