Re: [Phys-l] Weightless

[Hugh Haskell <hhaskell@mindspring.com>]

At 15:38  -0500 11/20/06, Richard Tarara wrote:
> I don't interpret what I feel in a jet aircraft accelerating forwards as
> weight as John seems to imply below.  I feel pushed back into my chair.
> Same with the rotor-ride I feel pushed into the wall--not heavy.  However,
> if I'm accelerated upwards I do feel heavier, and downwards lighter.  So,
> maybe I'm misinterpreting what John wrote, but in my experience the
> sensation of weight comes from vertical (or nearly so) forces with
> different sensations from horizontal forces.  I think we might both agree
> though that the sensation of weight in a rotating space station IS from the
> surface we are standing on pushing us towards the center of rotation.
I agree that the sensation that we call "weight" is generally limited 
to ones that are directed roughly parallel to our spinal column, or 
from front to back, if we happen to be lying down, but I suspect that 
those feeling are more culturally determined than by the physics.

I am thinking of the effects when riding one of the vertical circle 
rides at an amusement park. I am thinking particularly of those that 
go at constant speed, and at a speed so that you just come off your 
seat at the very top. The various sensations you feel vary widely at 
different points in the circle, although the centripetal force on you 
is constant, while the direction of gravity, relative to your body 
rotates through 360 degrees. Once the ride is moving at its constant 
speed the sensation one gets goes from, very heavy at the bottom (2 
gs) to "weightless" at the top (0 gs). In between the sensations are 
quite different. On the way up, although your speed is constant, you 
feel like you are slowing down, and on the way down there is a 
powerful sense of speeding up. The sense of speed change is much 
stronger than any sense of changing "weight," except near the top and 
bottom.

But picking up on the idea that weight is primarily limited to 
vertical forces, I would agree except that what is the perceived 
"vertical" varies with the situation. Once you have decided what is 
vertical that sense stays with you (at least temporarily) regardless 
of what orientation you occupy relative to that sensed direction, and 
that feels like "weight." But if you have been without outside 
reference for a few seconds that perceived direction can change 
radically, which is why we cannot fly airplanes in when we cannot see 
outside without reference to instruments which we have to use to 
orient the airplane with respect to the horizon, irrespective of what 
we "feel" is vertical, so if the airplane is upside down momentarily, 
but the force on the seat of your pants is still directed toward your 
head, you feel like you have "weight" (as in a barrel-roll) even 
though what is providing you with that reference is the centripetal 
force that is holding the aircraft in that horizontal spiral.

So our sense of weight is, I think, not quite so simple as you have 
expressed. Take away your external reference as to what is "up" and I 
suspect you will begin to feel whatever direction the support force 
on you is acting to be the (opposite) direction of your "weight," 
regardless of your personal orientation relative to that force.

Hugh
--

************************************************************
Hugh Haskell
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<mailto:hhaskell@mindspring.com>

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					Anonymous