Re: ATM Hogging Out -making flats????

["Tim Rickard" <trickar1@san.rr.com>]

>Everyone knows (especially Bob May) that gravity causes the disk on the
> >bottom to become convex while the one on top becomes concave.
>
> It seems to be great wisdom to say that gravity doesn't cause the
> concavity.  That's only partially true.
>
> The laws of physics show that gravity is PARTLY the cause.  Pressure?
> Certainly.  But will anyone swear that gravity will _not_ cause any
> pressure at the interface?
>
> It is obvious that gravity is partially responsible in cases in which an
> ATMer is advices _not_ to put any pressure on the mirror or tool with
> their hands.  Gravity is enough in that case.

Jim,

I think you are partially correct :) What happens when you exert force over
the center of the top disk with your hands?  Well, part of your mass is now
supported over the disks. The Earth exerts greater force on this larger mass
(gravity), but that force is still distributed over the same area. Hence,
grinding takes place faster.  As increase in mass, on either side of the
disk pair, will always increase grinding speed, although much more so when
applied to the top disk because of the much greater percentage mass
increase. This can be understood more precisely in terms of the
gravitational equation for two masses. It's not just partly gravity (mass),
it totally gravity.

The conceptual problem here is that people naturally think of themselves as
an independent third entity exerting force.  It is better understood for
current purposes as the opposite.  The Earth is attracting the mass of the
top disk and part of the mass of your body placed over it. Because the earth
is so big, the opposite attraction that the top disk/body exerts on the
earth has no effect on it. Thus, the earth slings you and the top disk back
and forth mercilessly. It is a two component system.  The earth and secured
bottom disk, vs. part of your body and the top disk (and there is the force
exerted by you on the earth which creates your motion).  There is no third
component. The earth wins; the top disk becomes concave.


Cheers,
Tim Rickard