Re: summary of weight

[John Denker <jsd@MONMOUTH.COM>]

At 07:01 PM 10/16/99 -0800, John Mallinckrodt wrote:
> Camp 1:
>
>   Weight is what you get when you integrate the law of universal
>   gravitation over all matter in the universe
>   (or, loosely, "weight is GM/R^2")

That's certainly not to my taste.   As stated it is inconsistent with
Einstein's principle of equivalence.  In particular it is inconsistent with
the well-known and reasonable notion that astronauts experience
weightlessness.

> Camp 2:
>
>   Weight is what you get when you multiply the mass of an object
>   by the acceleration of a freely falling object at the same
>   location *as measured within some chosen reference frame*
>   (or, loosely, "weight depends on the reference frame.")

That's the song I've been singing.

> Camp 3:
>
>   Weight is what you get when you multiply the mass of an object
>   by the acceleration of a freely falling object at the same
>   location *relative to the object*

This is a special case of Camp 2, and is therefore a valid option --
provided it remains an option and does not lead to intolerance of other
options which are equally valid and (depending on circumstances) might be
vastly more convenient.

>   (or, loosely, "weight is what a scale reads.")

That's waaaay too loose for me.
  a) Scales aren't expected to correct for buoyancy.
  b) Some scales measure mass, some scales measure weight.  The difference
only matters in non-pedestrian circumstances.  But if you are on Planet X,
fueling up your rocket to go home, it definitely matters whether you put in
a certain weight of fuel or a certain mass of fuel.  There will undoubtedly
be a weight limit (set by the mechanical strength of the rocket as it sits
on the pad) and also a mass limit (set by various other properties such as
in-flight accelerations).  You'd better not rely on some random scale you
brought from home (no matter how well it worked at home) until you figure
out whether it measures mass, weight, or some combination thereof.


______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com