Re: [Phys-l] football orientation in flight

[John Denker <jsd@av8n.com>]

On 05/22/2008 10:21 AM, Don Mathieson wrote:
> I dug in my files and found "Why Does a Football Keep its Axis Pointing
> Along its Trajectory" by Peter J. Brancazio  Physics teacher December 1985
> pps 571-573 (sorry I did not write volume number etc down)
> Basically conservation of angular momentum. He compares the football motion
> with a gyroscope.

Is that really what he says?  That's crazy.

The rule for free gyroscopes is called "parallel transport" and says 
that the axis of the gyroscope maintains its orientation independent 
of what is happening to the velocity vector, as shown here:
  http://www.av8n.com/physics/img48/parallel-transport.png

This follows from conservation of angular momentum.  

This is super-obvious and easily demonstrated in the case of a short, 
gentle toss of a spinning football.

If you want the ball to "follow its nose" (as CB put it) or more
precisely if you want the pitch attitude to follow the motion, 
than you need to supply a source of angular momentum, i.e.
torque times time.  

A short, gentle toss does not involve enough torque or enough time 
for there to be any significant departure from the basic parallel-
transport result.  An NFL Hail Mary pass evidently does.


=================
> > When a quarterback throws a long, arcing pass the point of the football
> > is intially upwards but at the end of its path the front points
> > downward.  Is the front pushed down because of Bernoulli's principle?  

It would be better to say "aerodynamics" rather than "Bernoulli's principle".
Bernoulli's principle is just a corollary of more important principles.

> > I
> > don't think that the rotation of the football is the primary cause
> > because when I throw the ball straight upwards the same effect does not
> > happen.

That's not a very highly sensitive check of the role of rotation.

A more sensitive check would be to throw (or kick) a ball with no
rotation.  I'll bet the non-rotating ball will not align its nose
with its velocity.