11 April 2012

Magnus at the Masters!

NOTE:  I wrote the following analysis on the evening of Sunday, 8 April 2012, not long after Bubba Watson won the Master's.  I've been delayed getting my analysis posted.

Bubba Watson's second shot on the second playoff hole of the Master's was a thing of beauty.  If you have not seen it, click here (go to the 5:13 mark on the video).  It not only won him the Masters, it was a great example of a golfer intentionally using the Magnus force to his advantage.  The Magnus force is the force associated with a spinning ball.  It helps explain curveballs in baseball and banana kicks in soccer.  A spinning ball whips air around its back in an asymmetric way.  What that means is that the air is whipped off to one side or the other instead of straight back.  Think of how a boat rudder works.  Turn the rudder to one side and water gets deflected, which makes the boat turn.  Newton's third law tells you that if a ball deflects air in one direction, the air must deflect the ball in the opposite direction.

Golfers use the Magnus force all the time.  Their club faces are grooved to help get the ball spinning.  On tee shots, a well-hit ball will have backspin.  The Magnus force associated with that backspin has an upward component, which fights gravity and helps keep the ball in the air a little longer than if the ball wasn't spinning.  Think about a good Major League fastball.  The backspin prevents the ball from dropping so much on the way to the plate.  A left-handed golfer like Bubba Watson will slice if he hits the ball in such a way that it has a component of spin that is counterclockwise (as seen from above).  What Watson did on the 10th at Augusta was go for a hook.  Seen from above, his ball had a component of its spin that was clockwise.  The ball thus came out of the trees spinning in such a way that the ball curved to the right.  Given that Watson was in the trees on the right side of the fairway, it was the perfect shot.  Most amateur golfers will hook or slice without trying to do so.  Watson showed us when a hook is a good thing.

The physics fun didn't stop with the ball in the air.  Once the ball hit the green, its spin caused it to take a right turn in front of the hole.  The ball eventually stopped rolling and a green jacket lay just ten feet from the hole.  Watson was then able to two-putt for the win because Louis Oosthuizen was not able to find the green with his second shot.  Bubba Watson can thank the Magnus force for the new addition to his wardrobe!

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