Re: What is weight? (was Re: Internal or external?)

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Mon, 11 Oct 1999, Ludwik Kowalski wrote:

> Interesting; if the weight is not the same thing as "the force
> of gravity" then what is it?

As I said before, I (and others) would have it be "what a scale reads"
or, more accurately, the magnitude of the vector sum of all
"nongravitational" forces.

> You would probably say "weight is zero in a free-falling elevator".
> Right?

Yes, as long as you aren't touching the sides.

> So what is weight?  Would you say it is another force?

See above.  Under this definition, weight is a force, but I don't think
I'd call it "another force."  I'm just not sure what you mean by that.

> It is the reaction force from the floor below me, and the spring above
> me.

Again I'm not sure what you mean by this.

> My weight is zero if the acceleration is g, it not zero if the
> acceleration is not g.

I'm not sure what you mean by "the" acceleration.  I think it is far
clearer to say, "My weight is zero if and only if I am in freefall."

> Acceleration becomes the "cause" of a force.  Something opposite to what
> we are accustomed to.  Is this what you want us to think about, John?

In a sense, yes, but only if you mean acceleration relative to local
freely falling frames.  That acceleration is in turn, of course, caused by
real forces.

I really don't think this is such a novel idea.  It is, after all, why we
say the astronauts are weightless; it is why we feel lighter or heavier
at certain points on a roller coaster; it is why objects have weight in
a rotating space station, etc.

Let me ask a question that has opened some eyes in the past: Suppose an
astronaut is launched in a rocket with an upward acceleration relative to
the earth's surface of 1 g.

a) How heavy does the astronaut feel?

b) What would you claim "the rocket's acceleration" to be?

Now, if that same rocket with the same astronaut used the same engines in
the same way in "deep space"

c) How heavy does the astronaut feel?

d) What would you claim "the rocket's acceleration" to be?

If this analysis doesn't grab you, then redo it with a rocket capable only
of hovering above the earth's surface.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm