Oh no! The definition of weight again!

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Fri, 12 May 2000, Rick Tarara wrote:

> I think you've introduced a 'third' definition of weight for which I would
> guess there is little support.  The choices as I understand them:
>
> 1)  Weight is the gravitational attraction of the earth on a massive object.
> (If the object is on the moon or Mars, substitute the moon or Mars for the
> earth.)  This definition leads to the discussion of apparent weight such as
> an 'apparent weightlessness in orbit.'
>
> 2)  Weight is the Net force as viewed within the frame of reference that
> causes the contact between an object and what supports it.

I know what it means to talk about a force *of* contact, but what does it
mean to refer to forces that *cause* contact?  For instance, what would
you point to as the force that "causes" the contact between my hand and a
baseball as I am throwing it?  Would it have something to do with the
muscles *in* my hand?  I sure wouldn't call that the ball's (or my hand's)
weight.  I just think this gets impossibly murky.

> For someone
> standing on a scale, this is still a downward force (since the scale
> actually reads the upwards force of the scale) ...

Given the third law, this may be a philosophical point, but I wouldn't say
that. I would say that the scale can only "read" a force that is exerted
*on* it.

> ... but includes the CENTRIFUGAL
> component due to the earth's rotation, and any effects due to other
> gravitational attractors.  Because this view looks at the centrifugal force
> as 'real' in the frame of reference where the object is at rest, then the
> actual weight in orbit is zero.

Hmm.  Suffice it to say I find this second definition completely
untenable.

> 3)  The now introduced definition of weight as the downward force of an
> object on it's support--a contact force.  I'm not sure what the source of
> that force is supposed to be if not (1).

I hope I have been consistent in previous discussions in making it clear
that this is my preferred definition of "weight."  For instance, it is, I
think, what we are referring to when we talk about "bringing our weight to
bear" on something.  I don't know why you should be confused about the
source of that force.  As with *all* forces that deserve the name, its
source is the physical object that exerts it.  When I stand on the ground,
the source of the force that I exert on the ground is me!  Indeed the
source is explicit in the phrase "the force that *I* exert on the ground."

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm