We're getting several subject threads going here, so I'm not sure which
one to respond to. The ideas below are in response to Denker,
Whatcott, Beaty and concern reaction engines, and pushing on the earth,
etc.
I have been getting a lot of good thinking from all contributors to
this discussion, so I hope we can sustain it a little longer without
getting on each others nerves.
I think we might be having a problem distinguishing between whether
something happens, as opposed to whether it must happen. Specifically:
(1) Does a hovering vehicle that is employing a reaction engine also
exert some force on the earth? Answer: typically yes. (2) Must it do
so in order to hover? Answer: absolutely not.
In this regard I strongly disagree with Denker when he says:
/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.
As others have pointed out, reaction engines do not require the earth
or any other object behind them upon which they push. The only thing
that needs to be pushed is the exhaust. Reaction engines work in deep
space regardless of whether the exhaust is aimed at earth, aimed at the
moon, or aimed at nothing.
When a helicopter hovers, or a rocket takes off, or an airplane flies,
the downward directed air (or exhaust) eventually exerts a force on the
earth, but that is not the force responsible for the flight/lift/etc.
The force responsible for the lift is the F = dp/dt in which the
momentum change is that which the vehicle imparts to the air or
exhaust. Whether the earth is present or not is totally immaterial.
John is just totally wrong that the only way to counteract the gravity
force between the earth and an aircraft is another force between the
earth and the aircraft.
In another post John says: "Rocket motors and machine guns work just
fine in a vacuum. Wings don't. Doesn't that make you worry that your
derivation isn't a reliable predictor of how wings work?"
No. That does not bother me at all. A rocket motor won't work in a
vacuum if you don't give it something to eject (i.e. fuel). A machine
gun won't provide thrust in a vacuum if you take away the bullets. But
that's exactly what you've done if you put a wing in a vacuum...you've
taken away its ammunition, so to speak.
Rockets are designed to carry the material (that they're going to
eject) with them, so they can still work in places in the universe that
are void of that material. Propeller driven airplanes indeed take fuel
with them, but the material that is going to have its momentum changed
as a result of that fuel; i.e. the material that is going to have its
momentum changed by the propeller (for thrust) and the material that is
going to have its momentum changed by the wing (for lift) is not
carried with the airplane. Hence, airplanes cannot work in regions of
the universe void of that material... air. That in no way implies that
airplanes are not acting as reaction engines.
If we cannot accept that lift occurs if a wing changes the momentum of
some air, regardless of whether the earth is present, then some of us
have some fundamental flaws in our physics understanding.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817