Re: [Phys-l] football orientation in flight

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
> Sent: Wednesday, May 28, 2008 4:10 AM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] football orientation in flight
>
>
>
> *) Before we "explain" "why" the ball keeps its angle of attack close
> to zero, we should look at the data again.  What I see is
>   -- The ball is nose-high during the first part of the flight.
>    So far so good.
>   -- The ball is nose-low during the final part of the flight.
>    All well and good.
>   -- However, in the middle part of the flight, it looks like
>    there is some nasty tumbling.
>
> Does the final nose-down attitude arise because of the tumbling?  In
> spite of the tumbling?  I have no idea.
>
> In any case, we'd better be careful, or we might wind up "explaining"
> a phenomenon that does not actually occur (angle of attack remaining
> close to zero).

Visit
<http://www.nfl.com/videos>
The "Featured" tab is selected by default.  This is the one you want.
In the "Browse Videos" menu on the left side of that page, select
"Can't-Miss Plays."  Look at the first replay on the 25th clip, the one
entitled, "Divisional Playoffs: Jackson TD catch  00:26".  The replay in
question starts at about the 12 second mark.  Then look at the first 10
seconds of the 144th clip, the one entitled, "Wk 13 Can't Miss Play:
Edwards TD Catch 01:08".

In both cases the realignment of the ball seems to occur fairly smoothly
without the nasty tumbling mentioned in the message (shown in part)
above.  Still, there is some wobble.

To me, at this point we have two possible explanations.  One involving
the circulation associated with ordinary airfoil lift and the other
involving the Magnus effect.  I'm leaning toward the former because the
spin rate doesn't seem to be that great, the relevant Magnus effect has
to do with the component of the center of mass velocity perpendicular to
the spin axis--a small component of the center of mass velocity-- and
the torque is dependent on the difference in the Magnus force on the
front half of the ball and the Magnus force on the back half.  Still, I
don't think we have enough evidence to rule out either one.

As mentioned earlier, a flight test involving a football launched nose
first with no spin would rule out the first explanation if the ball were
to travel with no tipping or the wrong kind of tipping.  Also, a clip
like the first one mentioned above (in which the ball is traveling
roughly toward the camera), probably with a greater frame rate, could be
analyzed to see if the kind of asymmetric wobbling associated with the
first explanation is occurring.