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A
typical
forward
pass
travels
in a spiraling motion along a parabolic
path. The term "spiral" implies that the football, during flight,
maintains a substantial spin about its long axis. A spinning
football possesses angular momentum and therefore must obey the law of
conservation of angular momentum: "If
no
external
torque
acts
on
an object, then the objects angular momentum
must not change." This essentially states that once the
football leaves the quarterback's hand, the angle of the football with
respect to the ground should not change. |
 http://footballphysics.utk.edu/balls/Motion.htm |
| However, you may have noticed that when a football is thrown with a good spiral, the nose of the football typically follows along with its parabolic flight path. This happens because the angular momentum of the football is changing in response to an applied torque. A force of air pressure on the leading surface of the football provides an aerodynamic drag that acts on the ball as if moves through the air producing a torque about its center of mass. The effect of such a torque on the spiraling football is to cause the ball to undergo a "gyroscopic precession." |
 Gyroscopic Precession is a fancy term that essentially describes the motion that a spinning top typically exhibits. |
 http://en.wikipedia.org/wiki/Gyroscope |
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According to
Professor Peter J. Brancazio in "The Physics of Sports", when a
football turns its axis to keep it aligned with its trajectory, it
represents a special case of gyroscopic motion. As it is being thrown,
a football's axis is aligned with its trajectory and there is no
initial torque. However, the trajectory of the football is curved so
that as the football proceeds, a small angle begins to form between the
spin axis and the tangent line to the trajectory. Since the
aerodynamic drag acts along this line ( the drag being always opposite
to the velocity vector), a small torque is created. The football
responds to this disturbance by turning, so as to reduce the angle (and
the torque) back to zero. Therefore the axis of the football mimics the
tangent line of its trajectory at that point (Brancazio, 273).
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