Re: Internal or external?

[Leigh Palmer <palmer@SFU.CA>]

> this would provide an excellent segue to the weight/weightless discussion,
> i.e. is weight what is measured on the scale or is it mg?  i.e. is weight
> synonomous with the "force of gravity" the force exerted by the earth on the
> rock in the above example?
>
> sorry for bringing this up again.

'sawright, but I get to put in my vote again.

The weight of an object is what is recorded on a scale supporting
the object when it is at rest. This operational definition can be
expanded to include moving objects. In that case the weight of the
object is the magnitude of the vector sum of the forces exerted on
it by the entities in contact with the object. This subsumes the
previous definition, of course, but it also accounts for the weight
of objects which are in accelerated frames, often called "apparent
weight" or "effective weight". I consider these latter "concepts" to
be gratuitous. They are unnecessary in my opinion, and they smack of
geocentrism as well!

In my scheme astronauts in orbit in the space shuttle are
"weightless" rather than "effectively eightless"; an astronaut
walking on the Moon has "a weight one sixth of his weight on Earth"
rather than "an effective weight one sixth of their weight on Earth"
as, for example, Hecht would have it.

Why is Earth weight so special that it doesn't have a qualifier
after it, and why isn't there a simple way to measure Earth weight?
Hecht insists that one must subtract the centrifugal force from the
gravitational force on an object to find its weight. He doesn't say
so in so many words, and he does say that there is very little
difference between weight and effective weight.

In my scheme I tell the students that the weight of an object at \
rest or in uniform motion on Earth's surface *can be modelled* by a
force which is proportional to the inertial mass of the object, m,
and that the proportionality constant, g, is conventionally called
"the acceleration of gravity". The Earth weight then is, simply, mg.
I point out that when one models physics on the surface of the Moon
one can do it in a similar manner, but one must use a different
value for g from that used on Earth.

Later on, when I introduce universal gravitation, I *could* show
them that weight can be modelled as the sum of the gravitational
force near earth's surface and the centrifugal force. I don't do
that, however. Instead I tell them that the sum of the gravitational
force and the support force (e.g. the normal force acting on an
object at rest on a bathroom scale) is a centripetal force which
causes the object to circle Earth's rotational axis onece per day.

What could be simpler? The title of Hecht's Chapter 5, "Centripetal
Force & Gravity", reads to me like these are two concepts of of more
or less equal stature. I can't understand how the author of such a
good optics textbook can have missed the mark so badly with his
introductory text. It shows the evidence of having required lots of
work, but in my opinion it is a poor effort physically.

Leigh