Not until after it passes the high point in the airfoil. Befor it
gets there, it is accelerated upwards.

Does the air split (top/bottom path) at the same level as it rejoins?
If the air splits at a higher altitude, then the air has to have a net
downward motion to get to the rejoin point.

When the air reaches the trailing edge it is back to where it started.

Is it? I think it's lower than when it started. It certainly is with
any appreciable AOA.

No, it is a way of looking at downrushing air that has never
contacted the wing.

It doesn't matter whether it contacts the wing or not. The contact is
by proxy (by contacting the other molecules of air that contact...the
wing.) You call it pressure. I agree. Pressure is ultimately
newtonian; I think we agree there too.

It's a floor wax. It's a dessert topping.

There is more to that. If this collision occurs in outer space, I
guarantee you that the center of mass will =not= quit moving.
But it will not move in a manner that conserves momentum.

Yes it will. What will not be conserved is macroscopic kinetic energy.

Make the room bigger. Make it an infinite room. At what point does
the fan continue to put momentum into the air mass continuously, and
not just during start up?

At the point when it's an infinite room. The bigger the room, the
longer it takes for a pressure equilibrium to occur. If we deal with
the earth's atmosphere and a propeller, the propeller pushes air back,
which alters the rotational momentum of the earth, in a manner equal
(and opposite) to the amount of rotational (around the earth's center)
momentum the airplane acquires. This could potentially happen until the
earth is spinning godawful fast (faster than the propeller could
handle). In practice we'll get tired of arguing before that.

So, are you saying that in the presence of the Earth there is no
net change in the momentum of the basketballs being thrown by the
dribbler and also no net change in momentum of the air molecules
accelerated by the wing?

Averaged over all basketballs and all air molecules, yes, because the
earth acts as a momentum transfer point.

Jose
--
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