Re: [Phys-l] Weightless (running around in circles)

[John Mallinckrodt <ajm@csupomona.edu>]

Joel wrote:

> I'm having trouble reconciling some of the statements made by John  
> D. and John M.  Whose opinions on the weighty matters I thought  
> corresponded;
>
> I think John M.'s definition of "gravitational force" below and  
> John D's definition of weight are the same??

I think that's right.  As I understand it from reading the first part  
of <http://www.av8n.com/physics/weight.htm>, JD would define both  
"weight" as mg where g is the local acceleration of a freely falling  
object.   That corresponds exactly with my preferred definition of  
"gravitational force."  On the other hand, I see no need to waste the  
perfectly good word "weight" by making it mean the same thing.  I  
would prefer to let weight be the MAGNITUDE of the gravitational  
force IN THE REST FRAME OF THE OBJECT.  It seems to me that this  
agrees far better with the use of the word in common parlance.  With  
these definitions, "gravitational force" is manifestly a frame- 
dependent vector, while "weight" is, even more manifestly, a frame- 
independent scalar.

For example, if I find that my weight is zero, then EVERYONE must  
agree that my weight is zero.  At the same time, they may find that  
the gravitational force on me is 200 pounds directed toward the  
center of Earth, or 3 tons directed toward Alpha Centauri depending  
only on the acceleration of THEIR reference frame.

> But I'm confused by what John M. is saying is frame dependent and  
> what isn't

I hope this helps!

John Mallinckrodt

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

and

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