Science World Blog How Does Spin Affect a Football?
How Does Spin Affect a Football?
Last Updated (Monday, 28 November 2011 23:04) Written by Raymond Nakamura
With the BC Lions winning the Grey Cup at home, football is in the air, and this got me thinking about footballs in the air. If you've played Angry Birds, you can appreciate the importance of the angle, initial force, and mass of the object in the trajectory of a projectile. With a football, another important factor is spin.
When Pigskins Fly
When you (assuming you are a well-trained quarterback) throw a football with your right hand (assuming you are a right-handed quarterback), the ball rolls off your fingertips (in particular your index) in a clockwise spin (assuming you are a quarterback with enough time to watch your beautiful throw before getting squashed by a defensive linebacker).
As the Football Turns
The spin results in angular momentum around the long axis of the football. This keeps the pointy end of a football facing into the wind, so it is more streamlined. A faster moving object requires more spin to remain stable, so throwing a football further (and faster) requires more spin. As the ball follows an arc through the air, the air pressure interacts with the spin, so the direction of the long axis of the ball follows the same arc. Without air, the angle of the axis would stay the same.
Downs
The spin also keeps air attached to the surface of the football, reducing drag. Less drag means the ball won't slow down as quickly. But the difference in drag is not symmetrical because of the spin, resulting in some drift in the direction of the spin. In other words, a football thrown by a right-handed quarterback will tend to veer right as it descends.
Who knew so much went into throwing a football? Hail Mary!
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I'm not sure about the effect of gravity in this one.