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Apparent weight

Date: Thu, 12 Feb 1998 09:50:39 -0800
From: Leigh Palmer <palmer@sfu.ca>
Subject: Re: REPLY TO ALL (about textbook misconceptions)

... we need a book, with explanations ranging all the way from the level
of mine to the level of David Bowman's.

Some of us may feel bad for being excluded, Leigh. Just kidding.

... I merely refer (and not at all obliquely) to the principle of
equivalence. If you claim that there exists something called "gravity"
you should be prepared to offer an operational definition by means of
which it can be measured.

Most elementary physics texts (and students) use the term gravity in two
ways: (A) as a phenomenon which manifests itself (or is responsible for)
weigth or free fall and (B) as a synonyme, sometimes for gravitational
acceleration and sometimes for m*g. I try to emphasize precision but I
am not very pedantic about it.

The principal of equivalence is often stated in textbooks but it is not
emphasized in the first semester. (Many of my students have hard time to
learn the difference between v and a). Same is true or the relative nature
of velocity; we do not always say "with respect to the road". Adding another
ambiguity to the term gravity is not desirable, in my opinion. The
gravitational pull (toward the center of Earth) in the orbiting shuttle, I
say, is about the same as at the sea level but the "apparent weight' is
nearly zero, as in a free-falling elevator.

According to the principle of equivalence there is no experiment
which can be performed which will didtinguish locally between the
presence of a gravitational field and acceleration of the frame in
which the measurement is carried out. ....

Calling the shuttle a microgravity (better would be migro-gee) is
entirely appropriate. ....

I do not need another noun in the first semester. Too many of them
already. The term "apparent weight" is good enough, I thunk.

Ludwik Kowalski