brian whatcott wrote:
Goofball_star_dot_etal wrote:

/snip/
No work is done on a plane in level fight at constant speed. Its
potential energy is not increasing with height and its kinetic energy
is not increasing with velocity. The lift is equal to its weight and
its drag is equal to the thrust. All the power ends up heating the
air, although initially some goes into increasing the kinetic energy
of the air you cannot get at this number by looking at the lift, as
you suggest, since kinetic energy and momentum are not the same thing.


Hmmmm. a mostly reasonable review - but the idea that force times
distance is not equal to work is somewhat radical, don't you think?


I said no work was done *on the (air)plane*. Since we only have the
airplane and the air, the work done by the thrust of the engine moving
the airplane through a distance all goes into the air as (kinetic)
energy or heat in its wake.

And if the force (usually called thrust in this context,)which was
provided to oppose the drag on the plane due to its airspeed, is
multiplied with the airspeed rather than some air distance - the work
becomes the power expended in opposing drag.

But you knew that, I'm sure

True.

- you were just trying to provide a gut feel
for the physics, huh? :-)

No I was responding to Roger's statement:

"The work done by accelerating the mass of air downwards is exactly
equal to the work required to keep the aircraft aloft"

I think he meant "force" but it is difficult to tell..