Re: [Phys-l] g...

[Jack Uretsky <jlu@hep.anl.gov>]

But, John, redundancy increases information content.  As a practical 
matter, it often increases understanding to have different names for 
the same quantity.  Force and weight are two words that are in common 
currency, and there can be some enlightenment involved in connecting them.
And we do this in many physics lab experiments when we use weights to 
supply forces for accelerating objects.  Experiment #1 in some labs is the 
weight falling along a tape that records the weight's instantaneous 
positions with sparks to demonstrate F=ma.
			Regards,
					Jack


On Sun, 19 Nov 2006, John Mallinckrodt wrote:

> I agree with Tim that one ought to treat established definitions with
> some respect, but doesn't it seem not only a shame, but needlessly
> confusing to use two different names--"force of gravity" and "weight"
> to refer to the same quantity?  At the very least, it seems to me
> that it would make more sense to let "weight" be to "force of
> gravity" what "speed" is to "velocity."  But at the risk of
> impudence, I will reiterate that my own preference would be to go one
> step further and make the weight of an object frame-independent by
> defining it as the magnitude of the gravitational force in the rest
> frame of the object.  Frankly, I think that corresponds most closely
> to its everyday usage.  For instance, we look at the astronauts from
> the frame of the Earth and say both a) that there IS a "force of
> gravity" acting on them and b) that they ARE "weightless."
>
> Tim folkerts wrote:
>
> > According to NIST (http://physics.nist.gov/Pubs/SP811/sec08.html) -
> > the closest we have in the US to an "official definition":
> >
> > "In science and technology, the weight of a body in a particular
> > reference frame is defined as the force that gives the body an
> > acceleration equal to the local acceleration of free fall in that
> > reference frame [6: ISO 31-3]. Thus the SI unit of the quantity
> > weight defined in this way is the newton (N). When the reference
> > frame is a celestial object, Earth for example, the weight of a
> > body is commonly called the local force of gravity on the body.
> >
> > Example: The local force of gravity on a copper sphere of mass 10
> > kg located on the surface of the Earth, which is its weight at that
> > location, is approximately 98 N.
> >
> > Note: The local force of gravity on a body, that is, its weight,
> > consists of the resultant of all the gravitational forces acting on
> > the body and the local centrifugal force due to the rotation of the
> > celestial object. The effect of atmospheric buoyancy is usually
> > excluded, and thus the weight of a body is generally the local
> > force of gravity on the body in vacuum."
> >
> > This makes it clear that
> > * "apparent" weight is "the" weight
> > * weight changes with different reference frames
> > * weight is NOT (G m(earth) m(object)) / r(earth)^2, but rather it
> > is this quantity minus centripetal effects.
> >
> > We can argue all we want about whether this is the best definition,
> > but it IS the accepted, standard, official US definition.  Wouldn't
> > it make more sense to get behind a single defintion, rather than
> > having each book and each instructor trying to present the
> > definition that he/she thinks is most familiar/useful/intiutive/
> > pedogogically sound?
> >
> > Tim F
>
> John Mallinckrodt
>
> Professor of Physics, Cal Poly Pomona
> <http://www.csupomona.edu/~ajm>
>
> and
>
> Lead Guitarist, Out-Laws of Physics
> <http://outlawsofphysics.com>
>
>
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

--
"Trust me.  I have a lot of experience at this."
		General Custer's unremembered message to his men,
		just before leading them into the Little Big Horn Valley