The only moment of real importance during a golf swing is impact. Every component of your swing, from how you grip the club, how you align yourself to the target, the infinite ways you can swing the club backwards, downwards and to finish, all serve to facilitate impact with the golf ball. But what is impact? It’s an obvious question, but one met with different answers depending on who’s giving the golf lesson. You’ll find it defined as anything from “the period a few feet before and a few feet after hitting the ball”, to a more exact “two inches before and four inches after hitting the ball” with anything in between and beyond.
In The Golfing Machine, its author, Homer Kelley, makes it very simple for us. Impact is simply “the meeting of ball and club”.(1) None of the arbitrary “a little bit before and a little bit after hitting the ball” for us. The exact location where the clubface first touches the ball is known as the Impact Point. When the ball leaves the clubface, this is known as Seperation. Everything between the club first touching the ball and Seperation is known as the Impact Interval.(1) This Impact Interval lasts approximately 0.0005 seconds.(2)
During this incredibly short time, the clubface collides with the ball, causing the ball to compress before leaving the clubface and returning to its original size and shape as it flies away. For a driver with a swing speed of 112.5mph during impact, the clubhead exerts 50,000Gs of force on the ball (that’s fifty-thousand times the force of Earth’s gravity) and produces an acceleration of 107,000,000 feet per second squared!(3)
The clubface will impart an initial direction and create spin around a particular axis of the golf ball during the Impact Interval, causing it to fly out on a certain path.(4)
Mr Kelley identified the “secret of golf” as “sustaining the Line of Compression” during the Impact Interval.(5) This "Line of Compression" is a concept we'll discover in later chapters.
(1) The Golfing Machine - Glossary
(2) The Search for the Perfect Swing, Cochran & Stobbs
(3) “The Golf Club - Ball Collision - 50,000G’s”, C.E. Scheie, Science & Golf
(4) The Golfing Machine - 2-D-0
(5) The Golfing Machine - 2-0