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Re: Apparent Weight



On Sun, 22 Feb 1998, Mark Sylvester wrote:


Another question:
This business of fictitious forces having no 3rd Law counterpart keeps
cropping up. Of course, in my seat in the accelerating 747 I feel myself
being pressed back into the seat, and the seat pressing back on me,

I find this descripton very difficult when I try to relate it to my own
experiences on planes: what I experience in that situation is the
pressure of the seat against my back, and I can only believe this is
pushing *forward* on me -- I don't see how the seat I feel pushing
against *my* back can be pushing me in the direction of the back of the
plane. Am I misinterpreting "I feel myself being pressed back into the
seat"?? It is exactly that "fiction" that something is "pressing me back
into the seat" that has no third law counterpart. The third law
counterpart of the push I experience from the seat is the force the seat
"experiences" from me, and that, of course, is toward the back of the
plane, but I in no way feel what the seat "feels."

but I
guess the case in question is the force which is accelerating the escaped
drinks trolley down the aisle to the back of the plane. OK, there seems to
be no 3rd law counterpart for this force.

There we're in full agreement.


So back to me seated in front of
my computer - I knock my coffee mug off the table and it falls to the floor.
In class (not the GR class) I would say that the Earth is pulling the
falling mug with force mg, and the mug is pulling the Earth back with
exactly the same force. Does this mean that mg is not fictitious after all?
...

In the context of Newtonian flat spacetime, the best Newton could do is
postulate such an instantaneous action-at-a-distance mutual interaction
between earth and the mug, pressing them together with a force mg down on
the cup and a corresponding (third law) force pulling up on the earth. (I
fondly believe that if Newton had thought of the beautiful idea of
curvature of spacetime, he would have invented general relativity instead;
I guess that's pretty much a tautology, since GR *is* the theory of
spacetime curvature and how it arises.) Once you do think of curved
spacetime, there is no need to postulate such a strange "force" acting
equally on masses of vastly different amounts. Both earth and the falling
cup are in the firm inertial grip of curved spacetime, moving with no
forces acting on either until they collide and the mutual
electromagnetic-nuclear interactions become important, pushing upward on
the mug and downward on the earth.

It may ultimately be possible to explain inertia itself (and hence the
curvature of spacetime) by guantum fields (Higgs bosons, Etc.?), and this
would be very satisfying, but we don't yet have any such theory of quantum
gravity that I am aware of.


Comments appreciated. Mark.



Mark Sylvester
United World College of the Adriatic
34013 Duino TS
Italy.
msylvest@spin.it
tel: +39 49 3739 255



A. R. Marlow E-MAIL: marlow@loyno.edu
Department of Physics, Box 124 PHONE: (504) 865 3647 (Office)
Loyola University 865 2245 (Home)
New Orleans, LA 70118 FAX: (504) 865 2453