Re: [Phys-l] Weightless

[John Mallinckrodt <ajm@csupomona.edu>]

On Nov 20, 2006, at 12:38 PM, 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.

Almost right.  It's not so much what we feel as what we perceive.   
Our vision plays a strong role in that perception.  I would say that  
it "misleads" us.

Indeed, it's interesting to note that flight simulators use the same  
visual misperception in reverse.  By tipping you backward while  
maintaining a constant visual field, they manage to make use of what  
I believe YOU would call "real" gravity in order to make you PERCEIVE  
that you are, instead, merely being "pushed back in your seat" as a  
reaction to forward acceleration.

> Same with the rotor-ride I feel pushed into the wall--not heavy.

In essence this is the same visual misperception.  (The only  
difference here is that there are additional real effects due to the  
rotation and the nonuniformity of the acceleration, but these are  
nonsubstantive differences)

> However, if I'm accelerated upwards I do feel heavier, and  
> downwards lighter.

What is "upward" and "downward" other than the direction that your  
body necessarily has to align itself when you stand in an erect  
position on a supporting surface?  Try standing in any conveyance  
that has a substantial horizontal acceleration.  Let the floor be  
angled so that you can stand on it as you would a level floor; let  
the acceleration be uniform and noiseless; and don some blindfolds!   
Then tell me you don't feel heavier.

> 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.

Of course.  And that is because "toward the center" IS the  
"vertically upward" direction in that situation.

This message tends to confirm for me that this is not, at root, a  
dispute about what we should tell students; I don't think there is a  
lot of dispute there.  It involves a fundamental disagreement about  
the nature of the world that we live in.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>