[drewitgolf]

Mr. Kelley pointed out the Four Periods of Acceleration in Chapter 8. Shoulder Acceleration (8-7, Start Down), Hand Acceleration (8-8, Downstroke), Clubhead Acceleration (8-9, Release) and Ball Acceleration (8-10, Impact).

But what is Acceleration? Acceleration is the speed of speed or the change in Velocity per unit time. Velocity, however, has a directional characteristic. If an object has a constant direction (moving in a straight line) but its speed is increasing we have Acceleration. If the object is moving in a circular motion with a constant speed the Velocity is changing because its direction is changing..and we have an Acceleration perpendicular to the motion (opposite to the force you would feel if you where in a car) or toward the center.
If both your speed and direction where changing, again we would have Acceleration.