On Wed, 8 Mar 2006 at 23:12:19 in message
, Jose
wrote:

The original question is ok (after all, in physics we use cylindrical
cows, frictionless surfaces, and point masses). But the comment at the
end is very misleading. The momentum is never "destroyed". It is
actually transferred to the wall, and thus to the earth. What they are
probably trying to say is that there is usually some rebound of the
water, and it sprays all over the place rather than becoming embedded
like machine gun bullets in sand... which would have been a better example.

I have got to be quick here - busy the next couple of days!

Their statement may be a bit sloppy but the fact is that because the
wall is very rigid and firmly fixed the ground an accurate calculation
of the force can be made by assuming the momentum is destroyed. The
effect on the earth is so small that it is minuscule compared to the
practical result.

Any changes to the entire earth as a result are insignificant.

Depends whether you are trying to understand the fundamental physics or
just trying to calculate an answer.

I thought the basic principles have been stated and stated! It is
whether or not we have a clear understanding of lift or not that seems
to be going around in circles. What happens to the earth a long way away
from the aircraft is, for all practical purposes insignificant and not
worth worrying about. It certainly does not prove that down wash has
nothing to do with lift for example as has been stated.
--
David CL Francis