Re: [Phys-l] Weightless

[Rick Tarara <rtarara@saintmarys.edu>]

As usual, there are two streams here.  How to approach things for Physics 
majors or in talking to other physics professionals, and how to approach the 
same things for H.S. students and General Education students (and perhaps 
even non-physics science/engineering students).  I don't really question 
anything JD or JM have said from the Physics major or professional physicist 
point of view--it is when dealing with my gen-ed class that I don't find 
their approaches AS useful as the earth frame, Newtonian viewpoint.  Now I 
do make it clear that our definition of weight is not shared by all, but is 
often used at this level of course--I make that even clearer with my 
Chemistry majors--but then go on to use what I've described in previous 
notes.

Rick  [off to grade gen-ed papers describing 5 Olympic events if the events 
were held in a domed, air-filled stadium on the Moon where I'm perfectly 
happy to have students take weight to be one-sixth the earth weight using W 
= mg !)

***************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN
rtarara@saintmarys.edu
******************************
Free Physics Software
PC & Mac
www.saintmarys.edu/~rtarara/software.html
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----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Monday, November 20, 2006 1:28 PM
Subject: Re: [Phys-l] Weightless


> 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!*