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Re: Lift of spinning objects.

Dwight,
The velocity of the ball relative to the air is of no direct consequence
here. It is the velocity of the air which matters - the pressure is less
where the airstream velocity is greater. The spinning ball drags an
adjacent layer of air along with it. With a backspin this gives a larger
velocity to the layer of air adjacent to the top of the spinning ball.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor
----- Original Message -----
From: "Dwight K. Souder" <crvhs_dks@CRESTVIEW-RICHLAND.K12.OH.US>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, January 18, 2001 6:56 PM
Subject: Lift of spinning objects.


The other day, I was reading in some old science book about flight. I
came
across an example I was a little confused with. The example was how a
spinning
ball was able to generate "lift" by giving it a backspin, in which I have
observed while playing volleyball or basketball. 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.
I understand that as the velocity of air increases, the pressure
decreases,
which ultimately leads to "lift". If a ball is thrown with no spin, the
relative velocity of the air on top of the ball is the same as the bottom
of the
ball (Right now, I'm neglecting vortexes that form around the ball). If a
ball
has a backspin, isn't the bottom of the ball have a higher relative
velocity to
the air? The way I picture it, it would be like someone in a car
traveling
50mph and throwing a ball 30mph ahead of him (with respect to the driver).
The
relative velocity of the ball to a "stationary" person would be 80mph.
Since the bottom of the ball is spinning into the wind, wouldn't it
have a
higher relative velocity in comparison to the top of the ball? I'm just
curious
how does a backspinning ball "float" while a forward spinning ball
"dives".

Dwight
Ashland, OH