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Re: Lift of spinning objects.



At 06:56 PM 1/18/01 -0500, Dwight K. Souder wrote:
The example was how a spinning ball was able to generate "lift" by
giving it a backspin,.... They authors gave an explanation that it was
due to the relative velocity of the top of the ball in respect to the
bottom of the ball.

There's a lot more to it than that.

I understand that as the velocity of air increases, the pressure
decreases, which ultimately leads to "lift".

OK.

If a ball has a backspin, isn't the bottom of the ball have a higher
relative velocity to the air?

In some sense it does, but it is a misconception to think that is relevant.

It is important to bear in mind that the air is a fluid, and has a pressure
and a velocity everywhere, not just at the surface.

Bernoulli's principle involves the velocity of the air, relative to the
velocity of the same air at earlier and later times. The velocity of the
air relative to the _surface_ of the ball is not directly relevant. It may
be indirectly relevant, to the extent that it imparts circulatory motion to
the nearby air. This circulation, superposed with the true airspeed of the
ball's overall motion, produces the velocity field that corresponds to the
pressure field that produces lift.

This is discussed in detail at
http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_circulation
and in particular
http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_spinners

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

More generally, explaining flight by analogy to the forces on a spinning
ball is problematic, for several reasons:
*) Not all balls play by the same rules. Balls with prominent seams,
such as baseballs and cricket balls, are in a different category.
*) The circulation near a wing is not produced by spinning the wing --
which begs the question of how circulation is produced (and how much
circulation is produced) by a wing.