So...about that plane on the treadmill...

Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...

The Rocket takes a small amount of matter and throws it at great speed
in X direction, getting and equal and opposite reaction in the opposite
direction Y... The prop grabs a large amount of air and throws it in X
direction, getting an equal and opposite reaction in Y... The principal
is the EXACT same.

Its the same principal as aircraft flying - while the common perception
is that aircraft fly because of pressure differential, 'really' they're
flying solely because they're imparting a downward force on the air
that is exactly equal the downward force caused by gravity- that is,
they're forcing exactly enough air downward to accelerate themselves
upward at 1 G...

The simple reality is that these are all various ways of stating the
EXACT same thing (and fundamentally, they all cancel out if you go back
down to the equasions)... Saying that an Aircraft flies via pressure
differential vs. flies via forcing air downward... or saying that a
prop or rocket flies via the same too manners (one might suggest that a
rocket flies with the same pressure differential as an aircraft, just
centered in a MUCH smaller area (inside the bell vs. outside, rather
than in front of the prop vs behind)... in the end, its the same net
result- mass gets forced backwards, I get forced forward... life is
good.


On Dec 13, 8:18 am, Jose wrote:
The thrust of the engine is not against the air. It generates
thrust as a Newtonian reaction to the prop moving air back, not
"pushing on other air." A rocket in space has nothing to push against,
yet it generates the same thrust as it did in the atmosphere.Actually, it's not that way. (but read carefully)

The thrust of a propeller engine is created when the propeller (an
airfoil) creates a high pressure area behind and a low pressure area in
front of the prop, as it pushes air back. The prop is pushing against
the air in order to do this. The air is constantly trying to get out of
the way, but it is not entirely successful, which leads to the pressure
differences. There's nothing funamentally wrong in saying that the
airplane pushes against the air to move forward. The prop (a part of
the airplane) is doing the pushing.

Rockets are different. The tail of fire coming out of the rocket does
push against the air (push the air out of the way to make room for the
fire), but it is =not= part of the rocket. If there were no air to
"push against", the rocket would work just as well, for that reason.
Where the rocket gets its thrust is the tail of fire pushing (the other
way) against the engine bell of the rocket itself. The rocket is
pushing against the fire, in essence. The fire is =not= part of the rocket.

Both cases can be viewed in the newtonian "action/reaction" paradigm,
but something has to push against something else in order to get the
thrust to happen in the first place. In a plane, the propeller pushes
against the air (to make the air go backwards fast and create thrust. In
a rocket, the engine bell pushes against the tail fire (pushing the fire
out, and the rocket forward). I suppose it may be clearer to say that
the expanding gasses of the tail fire push against the engine bell, but
the two are equivalent.

Although the expanding rocket gasses do push the air out of the way,
that doesn't help the rocket in any meaningful way.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

"EridanMan" wrote in message
ps.com...
Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...



At best, you could say that they accomplish the same thing. I don't agree
that they accomplish it in the same manner. Propellers, wings, and sails on
sail boats all work in the same manner. Whether you like to think of them as
redirecting mass or creating a pressure differential, they all do it the
same way. And none of them could do it without moving through a redirectable
mass - like air.

Rockets, on the other hand, carry their own redirectable mass with them, so
to speak.
-------------------------------
Travis
Lake N3094P
PWK

Incorrect.
You're missing a force in there.

Picture this:
Lay a tire on it's side on a frozen lake (close enough to frictionless for
this
example). Now wrap a rope around it a couple of times (like you would to
start an old outboard motor on a boat). Now quickly pull on the rope at a
tangent
to the tread.
You'll see two things happen.
1) the tire will rotate (Ok, that's pretty obvious)
but also
2) The center of the tire will move in the same direction that you are
pulling
on the rope.

The reason that the center will be displaced from it's original location
is the
same reason that the plane WILL move.

Your statement would be valid only under one condition.........If the
wheel, itself,
had no mass.


Hmmmm.. gotta think about that one. Not suggesting that you're incorrect but
just
wondering how the hub could act on the spindle if the bearing were truly
*frictionless*?!
Something has to push on something at some level. At what point are
frictional forces
described/replaced by more fundamental interactions? (maybe I need to
consult the great
oracle, wiki, to get the true definition of friction ;-)) On 2nd thought it
would seem that the
spindle WOULD be *pushed* backwards by photon pressure. (Holy crap! ... I
didn't want
to let THAT dead/alive cat out of the box.!) :-(

Also, by definition, the treadmill/conveyor will NOT MOVE AT ALL unless the
airplane/wheel hub
IS moving in the opposite direction. We're back again to the point of the
riddle which I believe
is to suggest that the airplane would take off regardless of what the wheels
or conveyor are doing.
(This is starting to sound more & more like the chicken and the egg.)

For Jose's question above, I think that by *frictionless* most people are
thinking of the hub &
bearings and not where the rubber meets the conveyor. If the wheel/conveyor
interface were
frictionless then neither the wheels nor the conveyor would turn unless the
problem can be interpreted
as the conveyor matching the hub's linear speed rather than the wheel's
angular speed. Either way
the plane takes off when it reaches sufficient airspeed.

Tony P.

muff528 wrote:
Hmmmm.. gotta think about that one. Not suggesting that you're incorrect but
just
wondering how the hub could act on the spindle if the bearing were truly
*frictionless*?!
Something has to push on something at some level.

A 'frictionless' surface can still exert a force normal (at right
angles) to the surface. So you can stand on a frictionless skating
rink and the ice will hold you up. It just won't exert a force
tangential to the surface so you may well slip and fall - at which
point you'll feel considerable force exerted on your body by the
frictionless surface, but it will be normal to that surface..

Similarly, the frictionless bearing can still exert a normal force on
the spindle/axle but it won't exert a torque since that would require a
tangential force.

The Rocket takes a small amount of matter and throws it at great speed
in X direction, getting and equal and opposite reaction in the opposite
direction Y... The prop grabs a large amount of air and throws it in X
direction, getting an equal and opposite reaction in Y... The principal
is the EXACT same.

Yes, inasmuch as it is action-reaction by Newton. The nit I was trying
to pick was that a propeller is viewed (correctly) as part of the
airplane, and the exhaust fire is also viewed (albeit incorrectly) as
part of the rocket.

The rocket pushes against the fire (in the nozzle, which =is= part of
the rocket). The airplane pushes against the air (against the
propeller, which is part of the airplane). These forces make the
vehicles go.

Although the rocket exhaust does in fact push against the air, the
exhaust isn't part of the rocket, and that "pushing" doesn't make the
rocket go.

Jose
--
"There are 3 secrets to the perfect landing. Unfortunately, nobody knows
what they are." - (mike).
for Email, make the obvious change in the address.

EridanMan wrote:
Jose...

not to be too much of an anal nitpicker, but actually, props and
rockets DO work in the same manner...

Not exactly the same. Rockets work in a vacuum, but propellors do not.

Matt