[Yoda]

During impact, the ball is 1) "deformed/flattened" (due to inertia, Newton's First Law of Motion, and its elasticity) and 2) "carried" briefly on the clubface (due to acceleration, Newton's Second Law of Motion) and then 3) "reformed and bounced off" (due to force and counterforce, Newton's Third Law of Motion).

Let’s slow the process way way down. Upon collision, the whole swing unit, including the clubhead, loses part of its momentum to the ball and decelerates (conservation of momentum). One portion of that energy is stored in the ball (which will be released upon reformation) as being deformed on the clubface and absorbed by the clubshaft and body, which all dampen the impact and give the golfer the sensation of a soft resistance on the grip. Other portion contributes to its acquired speed as being carried on the clubface by the clubhead before taking off (separation). Additional ball speed is then gained when the stored elastic energy is released upon the reformation of the ball (depending on the Coefficient Of Restitution of the ball) and bounces it off the clubface by pushing back the clubhead (like pushing yourself away from a wall moving in the same direction) causing its further deceleration. In short, the ball gets accelerated twice (being moved and carried and bouncing off) at the expense of the clubhead being decelerated twice (deformation and reformation of the ball). In contrast, a clay, which has a COR of 0, only gets accelerated once (without bounce) during the same process and launches at the same speed as the clubhead at separation.

Let say the clubhead's sweet-spot approaches the ball in-line (square to the clubhead path) at 100 mph. It's decelerated to 80 mph, the speed the ball picking up while being carried. So both the clubhead and ball travel at 80 mph at this stage. Assume the COR of the ball is 0.8, which means the ball is supposed to bounce squarely off an “un-decelerable” surface at 80 mph upon reformation while being initially deformed at 100 mph. Depending on how “strong” the clubhead is in resisting against the second deceleration by ball reformation (bounce) and the angle of impact (the more square 3-dimentionally, the more efficient bounce and deceleration), let say it happens to bounce off at 70 mph (which is ~88% of COR) and the clubhead gets pushed (backward) and slowed down to maybe 70 mph (needs to be measured to be sure) at separation. In summary, the speed of the clubhead goes from 100 to 80 and ends up at 70 mph at separation, while the ball gets accelerated from 0 to 80 to 150 mph at separation.

Note that the numbers used above are arbitrary and just for the explanation of the event of impact. The real numbers needs to be measured precisely under controlled experimental condition. As mentioned above, there are a lots of factors needs to be considered during the event. I'm not sure if the ball simply "acquires only 70 % of the Clubhead approach speed and 100 % of the Clubhead separation speed" as Homer described in the book.

There are, however, several things to be sure: with a given ball,

1. Higher approaching clubhead speed + squarer impact (sustained line of deformation) = more deformation (stored ball energy) and carrying period and carrying speed of the ball.

2. "Heavier" clubhead (pushing the closed door vs. pushing the door closed) + more stressed shaft (both relating to "LAG") + sustained acceleration (full release/sustained LAG beyond impact) + square impact + longer carrying period (including horizontal hinge) = better against secondary deceleration (pushing back) = more efficient bounce (preserving and releasing the stored ball energy).

It seems that the relatively softer and more elastic the ball, the easier for it to be deformed with better stability during carry.

Thanks for your great post, JJ. Very little is written about this fleeting Moment of Truth we call Impact. I found two of your points especially interesting:

1. "Depending on how “strong” the clubhead is in resisting against the second deceleration by ball reformation (bounce) and the angle of impact (the more square 3-dimentionally, the more efficient bounce and deceleration)..."

I have not heard anyone make these points since Homer Kelley in his 1982 GSEM Class. Producing this Three-Dimensional Impact -- especially the Downward element -- brings a more Vertical Clubface into Impact. The Ball tends to separate at the same Angle and with greater Compression. He maintained that this Impact, in turn, was one of the great benefits of the Right Forearm Takeaway -- the "Pick-up" as he liked to describe it. Such a Start Up produces a "more perfect circle" and a far better Downward motion through Impact than the flatter path of the Clubface (both in Start Up and into Impact) produced by the Shoulder Turn Takeaway. This strongly Downward Impact eliminates unintended Clubface Layback during Impact -- which is what happens when the Clubface comes "scooting in along the ground behind the Ball." Thus, it is directly responsible for a minimum of, in Homer's words, "Clubface Wobble," or as you said, "how 'strong' the Clubhead is in resisting..." the violent forces of the Impact Collision.

2. "It seems that the relatively softer and more elastic the ball, the easier for it to be deformed with better stability during carry."

I have three questions regarding your conclusions about the all important Ball characteristic of Resilience. First, does "relatively softer and more elastic" mean a higher or lower Compression Ball?

Second, does "easier...to be deformed with greater stability during carry" necessarily mean a higher COR, i.e., a 'better Bounce?'

Third, given your conclusions, is there an 'ideal Compression' Ball for all golfers?