On Tue, 7 Mar 2006 at 05:06:54 in message
, Jose
wrote:

Momentum is always conserved. If you see momentum disappearing, you
are not looking at the whole system. In the case of the land vehicle
propelled by a fan, the air blown back acquires momentum in one
direction, exactly balanced by the momentum that the vehicle acquires,
plus the (rotational) momentum (due to wheel friction) that the earth
acquires.

From a Physics book:

A jet of water merges from a hose pipe of a cross sectional area 5x
10^-3 m^2 and strikes a wall at right angles. Calculate the force on the
wall assuming the water is brought to rest and does not rebound.
(Density of water = 1.0 x 10^3 Kg m^-3)

After explaining the simple calculation which gives a force of 45N it
goes on to say; " in practice the horizontal momentum of the water is
seldom completely destroyed and so the answer is only approximate."

~~~~~~~~~~
Any changes to the entire earth as a result are insignificant. Closed
systems are adequate for most practical purposes.

All these arguments about the 'total system' are irrelevant to
considering the kind of problem we have here. As in many problems you do
not need to include the whole universe to get practical and accurate
answers.

In the same way including discussions about molecular velocities beating
on the sides of the aircraft is a mere distraction. At normal altitudes
these effects are negligible compared to the consideration of the air as
an incompressible fluid.

Some aircraft can maintain a 7g turn banked at the appropriate angle.
(81.8 degrees approximately). What happens to that 7 times pressure on
the earth now? The same calculations will give accurate figures of lift
only slightly affected by the small difference of speed between the two
wings and the circular path. The earth hardly comes into it for
accuracy except that it is gravity that is being balanced.

--
David CL Francis