On Thu, 1 Apr 2004 10:33:19 +0100, "JimB"
wrote:

I'm not proposing that the air 'has to catch up'. I'm just saying
that if it loses pressure, it's got to gain speed (or disperse
energy in some other way).

==================================

Let's try for an intuitive approach using a flat plate (your hand, for
example). Imagine sticking your hand out the window of a moving car
and "flying" it through the air as most of us have probably done as a
kid until our parents yelled at us.

If you hand is more or less parallel to the ground, you have wind
resistance (drag), but no lift. Tilt you hand slightly upwards and
now the wind strikes the bottom of your palm and forces it upwards
(lift). The reason lift is created is that your hand is deflecting
molecules of air downwards (change in momentum), and the resultant
force is upwards. It's simple Newtonian mechanics.