Effect of Rotational Direction on a Football.

  1. While learning about angular momentum in class my professor posed the question, "Why are their less left handed quarterbacks in the NFL than right handed?". He goes on to explain that in the case of a right handed quarterback the ball is given a clockwise rotation giving it angular momentum in the direction of its linear velocity (proven by the right hand rule) therefore increasing its velocity and increasing the possible distance thrown. Then he says that the left handed quarterback, who throws a counter-clockwise spiral, gives the ball angular momentum in the opposite direction of its linear velocity causes it to have a shorter trajectory.

    I simply could not believe this. It made no sense to me that the spin direction of the ball could effect its trajectory. (Assuming that each quarterback threw the ball exactly the same). I talked to him in order to get a better explanation and he basically just repeated himself. I've searched the internet for any material that may support this and have found nothing. I've even looked into the trajectory effects of the spin direction of bullets and found nothing.

    Could anyone explain to me why this is right/wrong?
     
  2. jcsd
  3. You were correct not to believe this load of crap that (s)he tried to feed you.
     
  4. D H

    Staff: Mentor

    It does, but not the way your professor thinks. A throw by a right-handed passer will drift slightly to the right. With a left-handed passer, the football drifts slightly to the left. This drift is small; a crosswind can cause a much greater deviation.

    Let's suppose for the sake of the argument that your professor is right. (He isn't. This is a "for the sake of the argument" discussion.) Unless the effect is very profound, he's still wrong! The ability to throw an 80 yard Hail Mary is very, very low on the list of what distinguishes a great quarterback from a lousy one. There are so many skills that are much more important than a desperation play that is only used at the very end of a half.
     
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  6. A.T.

    A.T. 5,755
    Gold Member

    But a left handed quarterback would use the left hand rule, not the right hand rule!
     
  7. Why would this be true?
     
  8. D H

    Staff: Mentor

    He was joking.
     
  9. I think A.T. was making a joke. But it was a joke with a point.

    The point is that the choice of whether to use the right-hand rule or the left-hand rule is completely arbitrary. If you redefine torque and angular momentum using the left-hand rule instead of the right-hand rule, the physics remains the same.

    The idea that the ball thrown by the right-handed quarterback will travel further because its angular momentum vector is pointed in the direction of its motion is complete nonsense. I'm not sure where on earth your professor got that idea, but it makes me question his or her fitness to be teaching introductory physics.
     
  10. I was surprise to discover that the teachers explanation isn't even an internet myth. No references at all. He/she must have made it up on his own.
     
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