AviationBanter - View Single Post - The Impossibility of Flying Heavy Aircraft Without Training

george

wrote:
Greg Esres wrote:
The disk is constantly transferring momentum to the air below it,
which is transferring it right back after bouncing off of the floor.

There is no momentum change here because there is no *net* force on
either the air or the disk. The molecules next to the disk have a
pressure equal to the weight of the disk below it and the actual disk
above it. There is no net force and thus no momentum change.

Pressure never equals weight for the same reason that voltage
never equals power.

To be precise, the upward force the disk is the difference
in the pressure below the disk less the pressure above, multiplied
by the area of the disk. The downward force is the weight of
the disk which is the product of the mass of the disk times
the local acceleartion due to gravity. The disk stops moving when
the two forces are equal.

Momentum = mass * velocity, and the vertical velocity of the air and
the disk are zero.

Force is defined as the time rate of change of momentum. Therefor
when the disk is neither accelerating nor changing mass there is
no force acting on it.

So what holds it up? ;-)

Lift pixies !