Re: [Phys-l] Weightless

[John Denker <jsd@av8n.com>]

On 11/20/2006 11:22 AM, Rick Tarara wrote:
> In the 'g' thread, we've (yet again) opened up the definition of weight 
> debate.  While I see certain pedagogical advantages to either of the two 
> main approaches, 

OK ....

> I would ask if the proponents of saying that one IS 
> weightless while in the space station can explain that from a Newtonian 
> viewpoint?  

That mis-characterizes one side of the issue.

My view (shared by a lot of other folks, including astronauts) is that
it is equally a blunder to say "IS weightless" or "IS not weightless".
The reference frame matters!
 -- The astronauts are weightless with respect to a frame comoving with
  the space station.
 -- The astronauts are not weightless with respect to an earthbound frame.

              *The reference frame matters!*

> Seems to me that there is only one force acting on the person. 

The gravitational force is frame-dependent.  Get used to it.

If you wish, you may choose to always analyze things using a reference
frame tied to the earth.  That's your choice ... but keep in mind that
others may choose differently.

Also note that Newton's laws are not exactly valid using an earthbound
reference frame; approximately, but not exactly.  To see this, analyze
the motion of a bowling ball, initially at rest high above the floor,
and then dropped.  Then (!) re-analyze it using a reference frame
attached to the bowling ball.  The equations of motion e.g. GmM/r^2
are symmetric with respect to interchange of earth <--> bowling ball,
so you should get the same answer ... but you don't.  In the second
case there will be gross violations of conservation of energy.

> If we call that the gravitational force due to the earth, then what (again 
> in the Newtonian viewpoint) is weight?  

Defining weight that way would be very unconventional and very impractical.
If I asked you to weigh something accurately, I would be astonished and
very disappointed if you corrected-out the contribution of the earth's
centrifugal field.  That would be beyond eccentric;  it would be perverse.

I also remind folks that our notion of horizontal and vertical include
frame-dependent terms.  Are you going to claim that the surface of a
liquid at rest in the laboratory is non-horizontal?  Are you going to
claim that objects spontaneously roll off a horziontal surface, because
of the earth's centrifugal field?  I hope not;  most people just level
the table in a frame-dependent way.  This is consistent with the convention
of measuring altitude with respect to mean sea level:  the sea is well
approximated by an isopotential if and only if we include the centrifugal
field along with the gravitational field.

Gravity, weight, et cetera are defined in a frame-dependent way.

              *The reference frame matters!*