Physics Question #2419

Steve Arkin, a 55 year old male from San Diego asks on December 9, 2004,

How is the spin imparted to a golf ball during the collision with a driver determined? The paradox is that many sources report "the gear effect" where the club recoils and imparts CCW spin (hook spin so the ball will curve to the left) on a toe hit, while an open club face (or a convex shape) will impart CW spin (slice spin which makes the ball curve left to right). I understand how side spin results from off-line club paths or non-square hits. I believe that these are in fact the dominant effect. My problem is that all drivers are made with a convex face. This means that a toe hit will have an open face, even on a perfectly straight hit. The effect described in the article should make the ball fade to the right. However, many sources claim that the reason for the convex face is to achieve the exact opposite: to make the ball curve back to the left on toe hits. The claim is that this is caused by "the gear effect" where (they claim) the club rotates back away from the impact and imparts counter-clockwise spin to cause a hook back to the left. I am not clear on the physics of the collision that can cause this to happen. I believe that the club will slow down after the collision, but I doubt that it actually translates backward. So is "the gear effect" real or a myth?

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The answer

Raymond Penner, Professor of Physics, Malaspina College, Nanaimo, BC answered on December 11, 2004

The gear effect is real. When the head of a driver is hit off-center it not only slows down but it also rotates about its center of mass. Its this rotation that causes the face of the club to rub across the golf ball and give it the side spin that is said to be due to the gear effect. The side spin due to the gear effect is opposite in direction to the side spin caused by the face opening or closing during the collision. i.e. For a hit towards the toe the club would open up causing a slice but the gear effect would result in a hook. Which effect dominates depends on how far back the center of mass is from the clubface and the specific impact point. In the case of the woods the center of the club head is far from the clubface and the gear effect dominates. The clubface is curved to compensate. The closer the center of the club head is to the clubface the less the gear effect and therefore the less the curvature is needed. That is why irons are typically flat.

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