The Role of the Batter's Grip in the Baseball-Bat Collision
Update, added 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.
The following material was put together prior to the Frazier home run.
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.
Rather than try to convince you of this here, let me instead refer you to an extensive discussion I carried on several years ago at one of the baseball blog sites. See the Inside the Book Blog for a discussion taking place on March 11, 2009, entitled "Everything you always wanted to know about the science of the bat/ball collision." It is quite lengthy and actually extends over several days. There are 64 different comments, mainly in the form of Q&A. Different people posed questions or objections and I tried to answer them. It was actually a very stimulating experience for me and that whole process helped me shape the arguments in a manner that most of the readers found convincing.
One of the references in that discussion is my paper, Dynamics of the baseball-bat collision. It is an academic paper I wrote that was published in the November 2000 issue of American Journal of Physics. See particularly Figures 13 and 15 and the discussion about them starting on page 987. I do not expect most of you to follow what was written in that highly technical paper. However, I include it for two reasons: (1) Some of you may actually be interested in it; and (2) It shows that my conclusion is the result of careful scientific study.
I would be remiss if I did not point out that the very first person to recognize that the grip doesn't matter was Prof. Howard Brody, who for many years was the world expert on the physics of tennis. In a seminal article Models of baseball bats published in the August 1990 issue of American Journal of Physics, Brody reports the results of his experiments and concludes the article with the following quote:
"...the concept of adding some fraction of the hand and arm mass to the bat mass to get an effective mass or striking mass seems to be contradicted by the results presented in this article."
Several years ago, Dr. Dan Russell wrote a web article entitled Does it matter how tightly you grip the bat in which he discusses at length the arguments, both theoretical and experimental, that the grip does not matter once the ball and bat are in contact.
More recently, two different videos have relevance to this topic. The first of these is the Falling Slinky Experiment, a YouTube video of an experiment done by Rod Cross. Watch the full video, especially the high-speed portion that will utterly surprise you. Also listen to Rod's explanation about what is going on and the relevance of his experiment to the issue at hand. Rod's experiment inspired me to do my own Vibrating Rubber Bat Experiment, in which I strike a rubber bat in the barrel and record what happens with high-speed video. The important point of this video is that there is a noticeable delay between when the barrel is struck and when the handle begins to move. That delay, combined with the very short time of the ball-bat collision, is at the heart of why the grip does not matter.
Finally, let me say that the "grip doesn't matter" conclusion is supported by lots of experimental data. These data are typically taken in a laboratory situation when a baseball is fired at high speed onto a bat that is initially at rest. The bat is supported at the handle; the impact with the ball is in the barrel. In these experiments, the speed of the rebounding ball is measured. The manner in which the bat is supported at the handle can be varied. The bat could be free to pivot about a point on the handle; it could be hanging freely and supported by a thin string; it could be gripped by someone either loosely or tightly; or it could be rigidly clamped. The results of the experiments can be summarized with this simple statement: The rebound speed is independent of how the bat is supported at the handle. Or, said differently, the grip doesn't matter.
