Why Throw A Spiral?

We thought about in the homework: does a rotating ball travel faster or slower than a non-rotating ball? If we look at the kinetic energy of the ball (which has a translational and rotation component), they we see that any rotational movement would only take away from the overall kinetic energy of the ball. This would therefore keep the ball from traveling as far or as fast as if the ball was not rotating at all. Then why do football players throw their balls with spirals? 

We have to take into account air resistance and how this affects the angular momentum of the ball. If we picture a ball tumbling through the air, its shape would continuously be changing and therefore so would the force of air resistance on the ball. This change in shape and force of air resistance makes it more difficult for the thrower and receiver to predict where the trajectory of the ball (and therefore where it will end up). However, if we have no external torques (such as the ball tilting), and the ball is spinning continuously, our angular momentum is conserved. 

Therefore, the putting spin on the football does not make it go farther or faster (quite the opposite), it merely makes the thrower more confident in placing the ball where they want it to go.