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Re: Apparent weight



A. R. Marlow writes:

<<However, exactly as in the case of the centrifuge, we are psychologically
tempted to create a downward force acting on us and pressing us to the
surface of the earth; it doesn't exist, can do no work and isn't
perceived by our senses, and so it is rightly said to be fictitious. In
the course of Einstein's development of general relativity (over a period
of several years), he realized this, and expressed it in the form of the
observation that such a "force" could be transformed away by a mere change
of reference frame. (This is never the case with a real force.) He then
went on to work out what gravitation is -- the effect of traveling
through curved spacetime. These real effects are as present on the space
shuttle or in a freefalling elevator as they are at the surface of the
earth. The main difference in the space shuttle is that no one is tempted
to create a fictitious downward force since they do not experience any
upward push from the earth.>>

Enlightening, now back to earth. First to quote Johnny Cash: I hear that
train a coming, its coming round the bend.

I once took a two quarter circuit analysis course. During the first quarter,
we ended studying RLC circuits. In the second quarter, Fourier and Laplace
transforms were introduced. The author posed the question, and I'm quoting
from memory. Why did we not introduce these transforms earlier? Then he
answered his question. Because it would be like using a locomotive to crack a
walnut.

If you were teaching a first quarter course in mechanics, how would you
approach the problems below? Remember that most of your students will not be
physicists, but many will be engineers. Also, they will transfer to
engineering schools that have certain prerequisites and that are perfectly
capable of teaching the physics they want their students to know.

Now I have an exam to write for an engineering statics class. While Newton's
name permeates the text, I don't recall seeing Einstein's name once. I wonder
how those bridges stay together?

Choose your model. How do you approach these problems? If you use a free
body diagram, what do you call the forces acting on the mass?

1. A mass is sitting on the floor of an elevator. The elevator may be at
rest, accelerating up, or accelerating down.

2. Same as Number 1, except the mass is on a scale in the elevator.

3. Same as Number 1, except the mass is hanging from a rope attached to the
ceiling of the elevator.

4. Same as Number 1, except the mass is on a chair and the chair is suspended
from ropes attached to the ceiling of the elevator.

5. The mass is in near earth orbit.


Bob Carlson