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Re: Apparent weight



On Sat, 14 Feb 1998, Leigh Palmer wrote:

John Mallinckrodt wrote:

Because I haven't heard anyone explicitly correct the misconception that
"microgravity" is just a weak "gravitational force" or that it is somehow
related to "apparent weight" allow me to do so: "Microgravity" refers to
the tidal effects which are gravity's only essential contribution to the
observable world and which are exceedingly small (hence, "micro") near the
earth. In a freely falling frame like the shuttle--which eliminates the
overwhelming and misleading appearance of a nearly uniform "gravitational
field"-- we find it much easier to observe these tidal effects.

Now I am confused. I explicitly stated that the principle of
equivalence applied *locally*. So far as I know there's no locally
observable effect of a tidal force. By "microgravity" I mean
"microgee", or gravitational fields being less than or of the order
of 10^-5 N/kg. As John correctly points out, the tidal effects are
observable in the space shuttle (see the cited article), but I don't
think they are the effects that are studied in what are usually
termed "microgravity" experiments.

Of course, you are right. I confess that I had always simply assumed that
the term "microgravity" was intended to acknowledge the small but
observable residual tidal imperfections in the otherwise gravity free
environment aboard a spacecraft. It takes only a few moments on the web,
however, to discover that the far more critical effects for many
experiments are the minor, transient accelerations of the spacecraft
caused by maneuvers and astronaut "collisions" with the structure.

Still, the term "microgravity experiment" is a little misleading since
most "microgravity experiments" are not *about* gravity and would really
prefer to be conducted in a strictly *zero* gravity environment. The game,
therefore, is primarily about reducing transient inertial forces to
tolerable levels.

John
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A. John Mallinckrodt http://www.intranet.csupomona.edu/~ajm
Professor of Physics mailto:ajmallinckro@csupomona.edu
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