Re: [Phys-L] kinematics objectives

[John Denker <jsd@av8n.com>]

On 05/09/2013 09:07 AM, Rauber, Joel wrote:
> I'll disagree that "the force of gravity" is a good euphemism for weight and vice-versa.
>
> Using them for euphemisms of each other is the main source, IMO, for
> the interminable discussion regarding the definition of weight that
> have occurred on these discussion lists ad nauseam since I first
> started subscribing.
>
> To whit:
>
> F_g = the force of gravity = GMm/r^2 pointing  to the center of the earth (in a earth context situation) according to many people.
> http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation
>
>
> Weight = mass times free fall acceleration as measured in a relevant reference frame according to many people
> http://www.av8n.com/physics/weight.htm#sec-def
> http://physics.nist.gov/Pubs/SP811/sec08.html#8.3 (see section 8.3)
> Am. J. Phys. 63, 105 (1995); doi: 10.1119/1.17990
> Am. J. Phys. 30, 387 (1962); doi: 10.1119/1.1942032

That's an interesting point.

To say the same thing another way: 
 -- The term "weight" is tricky because students were not born 
  knowing how to distinguish weight from mass.
 -- The term "gravity" is a source of misconceptions, because 
  it is ambiguous, as pointed out above.  This is, however, a 
  fixable problem.

We need to distinguish
 a) the framative gravity g@F         (for some frame F)
 b) the barogenic contribution δg_M   (for some mass M)


Let's be clear:  I strongly recommend writing the law of universal
gravitation in terms of _delta_ g:

                      G M
          δg_M(r) = ------- r̂                 [1]
                      r^2

In general, δg is the _difference_ between the gravity at location 
R+r and location R.

The quantity δg_M is one contribution to this difference, namely
the contribution due to a mass M concentrated at location R.
This is not the only contribution to g, nor even the only
contribution to δg.

This is a long but not venerable tradition of using the word
"gravity" to refer to both the framative gravity g@F and the
barogenic contribution δg_M.

For details on this, see
  http://www.av8n.com/physics/weight.htm#sec-various-notions

I apologize for incautiously using the ambiguous term "weight".

However, when talking to students on the first day of the
introductory course, this is perhaps forgivable.  For mundane
activities on the surface of the earth, the local framative 
gravity is "the" gravity.
   It is only later, when the law of universal gravitation
   is introduced, that it becomes necessary (or even possible)
   to discuss the ambiguities.