Which way would a dirty golf ball turn?

14 February 2010

Question

In an article in the December 2009 golf digest, they were talking about a ball with mud on the one side of the ball. Now I would assume with the extra weight on, lets say, the left side, the ball should move toward its left side when hit? They say the opposite. They say mud on the left side will cause the ball to move right? Please explain this to me?

Answer

What you're interested in here is the forces on the ball. If you've got air going past the ball the only thing which can touch the ball is air (other than gravity - which we're going to ignore because it's not very interesting in these circumstances). So you've got air getting past the ball and so if there's going to be a force on that ball the air's going to get deflected one way or the other. If the with air is deflected to the right then there's going to be an equal and opposite reaction on the ball it will go to the left and vice versa. Now what's going to happen if you put some mud on the right hand side of the ball? You've got air going over both sides of the ball - on the left hand side of the ball, the ball is just as normal and the air's going to stick to the edge of the ball and come around as far as it would do normally. On the right hand side of the ball you're going to get a whole lot of mud, it's going to be rough so the air is going to become turbulent and it's actually going to leave the ball earlier. So the air coming from the left is going to stick on the ball longer than the air going to the right. Which means overall the air's going to start moving to the right which means overall the ball is going to move to the left.

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