I was browsing a sailing newsgroup the other day when someone
pondered out loud about the effects of a heavy ship sailing
over a shallow bay.
"How much of the seabed feels the effect of the battleship
sailing above it?". One of those deceptive mind experiments.
I expect teachers have often heard this question.
And I expect you respond that all the seabed feels the increased
pressure - but only the increase in water pressure provided in the
just the ordinary way, by the increased head from the rise in water level
when the ship was launched.
It reminded me though, of the storm in a tea cup on this list when
someone was arguing about the extent of the downflow from a passing
jumbo jet. Did it/didn't it reach the ground?
Of course it does/doesn't - sort of thing....
So let me share a modest insight: though the down thrust from the
wings of a big airplane never directly reaches the ground
(from reasonable heights of several thousand feet) the answer to
where its weight is reacted is of the same kind as the one for the
destroyer sailing over the sea bed.
Yes! the ground carries the airplane's weight - but the mechanism is
just the ordinary one - the pressure of air in this case.
The jumbo's downwash (like the work put into Joule's paddle)
washes out in a very modest temperature rise - which if enough jumbos
persist in flying, raises the air pressure felt on the ground over a
considerable area.