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Re: A list of textbook misconceptions



On Wed, 11 Feb 1998, Leigh Palmer wrote:

5) Gravity in space is zero (or very small, e.g. NASA's 'microgravity')

Gravity inside an orbiting shuttle is very small compared with gravity
at the same point at rest with respect to Earth. The misconception is
that one can ascribe a unique "gravity" to a point in space. The
gravitational field is relative (just as are the electric and magnetic
fields). It depends upon the frame of reference of the observer.

So, should we tell students that when a pencil falls from my hand, it
stops experiencing gravity? Or, that when I jump up and down on the
earth, gravity varies enormously? GR descriptions may be more general,
but acceleration caused by a 1/r^2 force is a common description as well.
Mixing the two together causes problems. Do you think it acceptable to
state that the pencil accelerates downwards because it is experiencing
gravity? If so, then we'd better stay consistent and say that the shuttle
also takes a curved path around the earth because it experiences gravity.

I think that much of the problem is from a K-12 description colliding with
a graduate-school description. Newtonian gravitation is used to explain
falling pencils, while GR is used to justify the term "microgravity". One
author says that gravity at earth's surface is a constant, another says
that there is no single gravity at a single point on the earth. How
should K-12 students (and educators!) interpret this contradiction?

It is my experience that many people believe that gravitational attraction
ends at the top of the atmosphere, and this is why the shuttle occupants
are "weightless." NASA's term "microgravity" unfortunately supports this
mistake. Micro-gee would be better ( where "gee" is a unit of
acceleration.)

A fun calculation: figure out the force applied to the space shuttle when
sitting on earth, then when raised to 500KM orbit altitude.

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