The Impact Interval

 Dr Aaron ZickBy Dr Aaron Zick (Physicist)  

 Dr. Aaron Zick, a Ph.D. in Chemical Engineering from Stanford University, Physicist and President of Zick Technologies, and a golf swing physics expert who really understands the science behind how things work in the golf swing.

Hi John,

I'm glad you enjoyed the anti-summit and the new ideas that were presented there.

Let's First Define some Terms.

First Contact - will mean when the clubface first contacts the ball,

Separation - will mean when the ball leaves the clubface,

Impact Interval - will mean the time between First Contact and Separation,

Maximum Compression - will mean when the force between the clubface and the ball is at its maximum.

Also, assuming a right-handed golfer, "open" (for either the clubface or the path of the clubhead) will mean pointing to the right of the target, "closed" will mean pointing to the left of the target, "square" will mean pointing at the target, and "closing" will mean rotating counter-clockwise.

Now, suppose the face and the path are both open at first contact and square at separation. Homer Kelly claimed that the ball will be launched in the direction of the club face at separation, which would be straight at the target in this case, but he was wrong.

The ball's initial trajectory will actually be to the right of the target. The force between the face and the ball is zero at both First Contact and Separation. Everywhere in between, both the face and path will be open, so the Forces on the ball will always be pointed to the right of the target.

Thus, all acceleration will be to the right (Acceleration = Force divided by Mass), and therefore, the final Velocity will be to the right (Velocity = the integral of Acceleration over Time).

If the face and the path are closing at the same rate (as they would during "Angled" Hinging), the Forces will only be "normal" (i.e., perpendicular to the face), as opposed to "Tangential" (i.e., in the plane of the face), so there will be no side-spin imparted on the ball and its flight will be a straight push.

On the other hand, if the face is closing faster than the path (as it would during hinging that was more "Horizontal"), then the face will always be more Open than the path (until Separation, when the Forces are zero), so a clockwise spin will be imparted on the ball and its flight will be a push slice.

Look no further than a well-executed top-spin shot in tennis or table tennis, where the impact interval lasts much longer than in golf and the aiming directions can change much more noticeably between First Contact and Separation. In both of those cases, the racket or paddle might be aimed almost straight down at separation. Kelley would conclude that the ball should dive straight into the ground or table, but we've all seen it fly over the net instead.

In order to send a golf ball straight at the target, there must be no right or left component to theVelocity, which means that the right and left components of the Acceleration (and hence, Force) must integrate to zero.

Furthermore, all side-spin-inducing Tangential Forces must also integrate to zero. In other words, all "pushing" Forces must be counter-balanced by "pulling" Forces and all "slicing" Forces must be counter-balanced by "hooking" Forces. The only way to meet these criteria with a normal swing (whose clubhead path and clubface are both closing during the impact interval) is for the face and path to both be open at First Contact, closed at Separation, and square somewhere near the maximum Compression Point.

In fact, if the Forces are perfectly symmetrical during compression and decompression (a very reasonable assumption), then the face and path should be perfectly square at maximum compression and symmetrically open and closed at First Contact and Separation, respectively (at least with Angled Hinging, which would generate no side-spinning forces; a very slight adjustment might be required for any other form of hinging because of the non-symmetrical sliding friction that would occur immediately after first contact).

How much of a push would be produced by a swing whose clubhead path and clubface were both square at Separation instead of at maximum compression? For Angled Hinging, the initial trajectory would be approximately in the direction of the path and face at maximum compression, about halfway between first Contact and Separation.

That would be about one-third of a degree to the right of the Target Line. There would be no side-spin, so the ball would be pushed about 1.2 yards to the right on a 200 yard shot (pretty negligible).

For Horizontal Hinging, however, the clubhead path would still be about one-third of a degree open at maximum compression, but the clubface would be about two-thirds of a degree open. Depending on the coefficients of sliding and rolling friction, the initial trajectory might be about 0.6 degrees to the right (or 2 yards on a 200 yard shot), but there would also be an induced slice.

According to Cochran and Stobbs in "The Search for the Perfect Swing" (1968), a 200-yard shot will slice approximately 7-8 yards for every degree the clubface is open to the clubhead path, so we should expect about 2.5 yards of slice, making the total error about 4.5 yards (still relatively small, but not completely negligible).

In Summary,

To hit the ball straight with no slicing or hooking, the clubhead path should be slightly open at First Contact (about a third of a degree, typically) and approximately the same amount closed at Separation.

The clubface should also be slightly open at First Contact (about one- to two-thirds of a degree, typically, depending on the Hinging) and approximately the same amount closed at Separation. Any deviations will cause the ball to start and/or drift somewhat off-line.

Hoping this helps John,

I'm glad to help further the understanding of the science behind the sport.

Good luck,


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