Nathan: The role of the batter's grip in a batted ball collision

From SABR member Alan Nathan at The Physics of Baseball on May 28, 2012:

On May 27, 2012, Todd Frazier hit a home run with his hands barely gripping the bat. See the animation below and/or click here for the actual video (starting at about the 32-sec mark). Note in these images that the top hand is not in contact with the bat and the bottom hand is very loosely in contact with it at the moment of collision with the ball. So, the bat is essentially a free object. Nevertheless, the ball is hit for a home run, demonstrating in dramatic fashion that the batter's grip plays no role in the ball-bat collision. Viewers with a sharp eye will see the bat actually vibrate after it leaves Frazier's hands. Thanks to Dan Russell for creating the animation.

In my many talks on the physics of baseball, I often bring up a point that is not very intuitive to baseball fans. Namely, for a typical ball-bat collision in the barrel of the bat, the batter's grip plays absolutely no role in determining the ultimate fate of the ball. The batter's primary job is to get the barrel of the bat on the ball squarely with as high a bat speed as possible. Of course, the batter's grip plays a very important role in making that happen; i.e., in getting the bat to the right place at the right time. But, once bat meets ball, the grip is not longer important. In fact, the batter could just as well let go of the bat just prior to meeting the ball and it would make no difference to the ball (although, it certainly would matter to the bat!). Here is the basic physics in a nutshell. When the ball impacts the barrel of the bat, a transverse wave is created in the bat that propagates down the axis of the bat toward the knob, reflects off the end (or the hands, if they are there), then propagates back to the barrel again. Here is the key point: The ball-bat collision is very rapid, of order 1 ms or less. By the time the transverse wave reflects back to the impact point on the bat, the ball has long since departed. So nothing on the handle end of the bat could possibly influence what happens to the ball: not the size or shape or weight or even the hands.