Re: [Phys-l] football orientation in flight

[John Denker <jsd@av8n.com>]

Here is a NY Times article:
  http://query.nytimes.com/gst/fullpage.html?res=9D07E0D61539F931A35752C0A960958260&sec=&spon=&pagewanted=all
by WARREN E. LEARY  Published: January 2, 1996

The main part of the article is annoyingly tantalizing;  it hints at
the physics but doesn't give enough detail so that you can actually
follow the physics.

I was however glad that I read the article to the end, where it says:

> One football mystery that still needs a formal explanation is why the
> long axis of the ball changes direction during a throw to follow the
> path of the trajectory of flight, Dr. Brancazio said in an interview.
> In theory, the angular momentum of a spinning football should remain
> constant if there is no external force pushing it down. So a football
> thrown up at a 60-degree angle should come down at the end of its arc
> with its nose pointed up at 60 degrees, instead of pointing down
> along the path of the flight.
>
> Dr. Brancazio said he believed that a football tipped its axis to 
> keep it aligned with its trajectory because of a special type of 
> gyroscopic motion seen in a rapidly spinning, and highly stable, toy 
> top. As a small angle opens up between the ball's spin axis and the 
> trajectory, he said, aerodynamic drag creates a small torque. The 
> football, acting like a spinning top, must respond to slight 
> disturbances like the small torque by righting itself and keeping its
>  axis aligned with the trajectory all the way to the end, he said.
>
> "When anyone talks to me about formulas explaining how footballs act,
>  I ask them how their numbers deal with this," Dr. Brancazio said. 
> "It's not clear why the spin axis tips over in flight, and this is a 
> good test."

So ... if we can't think of a good explanation for the pitch
change, at least we're in good company.