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Re: Apparent weight



What Roger and I, and I think Leigh are advocating and pursue (in our
introductory classes; for advanced classes I talk more along the lines of
John and freely falling frames); is contrary to what most books say. I
will presume (perhaps erroneously) to speak for them here. They basically
define weight as the force of gravity acting on the object, i.e. mg . The
arises when you consider a shuttle occupant. Every student *knows* they are
weightless up there. Following the book's typical definition, this would
seem to imply that the force of gravity does not act on them. But how do
reconcile that with the need for a net centripetal force (as seen in the
frame of the stars, and NO , I don't want to start discussing inertial and
non-inertial frames again) to cause the orbiting motion of the shuttle and
its contents.

The way I approach the subject avoids those problems, and probably
emphasizes the subjective nature of the idea of weight; which may be a good
thing. Of course we pay the price of necessarily defining weight through an
operational definition; but this seems to cause no difficulties, as the
students are already quite familiar with operational definition; most of
them having weighed themselves before; and if not we have scales in the lab
that they are forced to use.


<< Once our students get this separation of weight from the mg force, then
they are far more successful with solving dynamics and statics problems. >>

This is my experience as well; I might add as well, they it removes some
incongrueties regaring centripetal forces and circular motion; and aid their
understanding. This is a notoriously difficult concept to across, and is a
prime motivator for me in defining weight the fashion that I do.

Joel

Apologies to Roger and Leigh, if I have misrepresented them in any fashion
in the above.
----------
From: Raacc
To: QuistO; RAUBERJ; phys-l
Subject: Re: Apparent weight
Date: Saturday, February 14, 1998 2:19PM

In a message dated 98-02-14 13:35:36 EST, you write:

<< Once our students get this separation of weight from the mg force, then
they are far more successful with solving dynamics and statics problems. >>

This is surprising since most texts I've used define weight as mg. I've
just
looked through these texts that all define W = mg.

Physics, Resnick/Halliday/Krane

Physics for Scientists and Engineers, Tipler

Fundamentals of Physics, Halliday/Resnick/Walker

Engineering Mechanics Statics, Hibbeler

Vector Mechanics for Engineers Statics, Beer/Johnston

Physics for Scientists and Engineers, Serway

What texts define it differently and therefore make statics and dynamics
problem solving easier?

Bob Carlson



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