Posted by: JJA () on Wed May 12 13:11:12 2004

Followed the site for a while, so I know what Jack’s explanation would be, but it would be great if JJA, Nyman or one of the others versed in the sciences gave us the technical explanation (how it fits into Adair’s and other’s physics model) of what is causing Sosa’s bat to become a blur well back in the swing (starting in frame 10) of the following clip:
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Hi,

This is a very difficult question to answer in a thread. I refer you to Dr. Adair's full chapter on this, including many informative graphs including his modeled swing path, which by the way, is not that far from a circular path. I'll give you a couple of ideas to think about.

Dr. Adair's model is that there is little torque applied to the bat, but instead a linear force is applied in the direction of the bat handle. Think of an arrow sticking out the bottom of the bat. That represents the direction of the force applied by the hands on the bat, in his model. Swing a bat and feel the pressure on the bottom of the lead hand near the knob. This is the force that Dr. Adair is describing that is being applied to the bat. As an aside, this is why one can swing a bat with the lead arm only with significant bat speed. It does not require differential forces (i.e., torque) of the hands to generate bat speed.

What I believe the root conceptual difficulty here is that this force is applied in a direction that is continually changing. Indeed, as Jack correctly pointed out - and I give him full credit for being the first to make this observation - the hands have a (nearly) circular trajectory. Strange, isn't it. A force that is being applied in a circular direction. Isn't that identical to a torque being applied to the bat? From a purely physics viewpoint, no. The video of Sosa is entirely consistent with force being applied primarily in the direction of the bat.

Why this question is so difficult to answer is that, despite attempts to ridicule this line of reasoning, it involves biomechanics of which I do not purport to be an expert. Between frames 1 and 10, his shoulders have rotated significantly. (Difficult to tell, but it looks in excess 40 degrees.) This rotational energy in the upper torso obviously gets transformed into the arms which then transform energy into the bat but the details of how this is precisely done is beyond my knowledge.

I actually think of this problem as a two link robot, which Nyman displayed so superbly in a recent post. That example shows how rotational energy of the first link (body) can generate significant linear velocity on the second link (read bat speed) when no torques are applied to the second link (bat via wrists). Although it is certainly a gross approximation, I think it is a very useful one and probably the best example of describing Dr. Adair's model that I can think of without getting too mathematical.

-JJA