Re: To hover, a reaction-motor pushes on the earth?

[William Beaty <billb@ESKIMO.COM>]

On Wed, 18 Aug 1999, John Denker wrote:

> At 02:02 AM 8/18/99 -0700, William Beaty wrote:
> > On Sat, 14 Aug 1999, John Denker wrote:
> /jsd/ So does *everything*.
> /jsd/ Gravity is a force between the earth and the aircraft;
> /jsd/ the only way to counteract it is a force (indirect or otherwise)
> /jsd/ between earth and aircraft.
> /jsd/ The only question is how indirect it is going to be.
> > Everything flys by pushing against the earth?  I strongly disagree.
>
> I stand by my assertion that in a closed system, the wing pushes indirectly
> against the earth.  The only question is how indirect it is going to be.
> My assertion is a simple consequence of Newton's laws.  If we can't agree
> on this, we have nothing further to say to each other.

Perhaps I am confused by the meaning of the word "push".  If you assert
that the wing pushes indirectly upon the earth, then by Newton's laws
there is a (indirect?) force-pair between the wing and the earth.  Are you
saying that there is a force-pair between the wing and the earth?  Let's
be clear.  By "force pair" I mean this:

  The wing creates a downwards force upon the earth, and the earth creates
  an *EQUAL* and opposite force upon the wing.  If the wing pushes down
  upon the earth with 10,000Nt, then the earth pushes upwards upon the
  wing with 10,000Nt.

Would you agree with the above statement?  If so, then a very important
question arises.  How can the distant surface of the earth push upwards
upon that tiny wing up there?  The moving air from the wing pushes down on
the earth, but how can the earth communicate an equal upwards force?  And
if the earth does NOT push upwards upon that wing, then why does the wing
stay up?

If the wing pushes down on the earth, but the earth doesn't push upwards
upon the wing, then Newton's third law is violated.  Either you must
explain the mechanism by which the earth's surface reaches upwards and
applies force to an aircraft, or you must admit that there is no
force-pair between earth and aircraft.  If there is no force-pair between
earth and aircraft, then this statement is wrong:

> /jsd/ Gravity is a force between the earth and the aircraft;
> /jsd/ the only way to counteract it is a force (indirect or otherwise)
> /jsd/ between earth and aircraft.

It is wrong because there *is* a way to counteract gravity without
creating a force-pair between the aircraft and the earth.  That method
involves the physics of reaction-motors.  Throw mass downwards, be lifted
in return.  Mass hits the earth, but earth does not lift you, and so there
is NOT a force (indirect or otherwise) between you and the earth.  Simple.


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