Posted by: ray porco () on Thu Aug 25 18:49:17 2005

> >>> Re: "weight back" Jack Mankin Fri Aug 19 18:59:56 2005
>
> Hi Grc
>
> Good questions. I also would find it helpful to have a clear definition of “weight shift.” If the batter takes a short stride but the center of his body mass remains stationary (zero forward velocity) through rotation, was there a forward weight transfer? Since momentum is the product of mass with velocity, there would be no momentum to transfer if the velocity is zero. – Or, is “weight shift” just to indicate which leg is most responsible for supporting the body’s weight at different points of the swing?
>
> Jack Manklin
> ##
>
> No one else has a definition of “weight shift”?
>
> Do you have one Jack? <<<
>
> Hi Ray
>
> As I pointed out above, for a “weight shift” to occur that produces linear momentum, the center of mass (the spine) must attain velocity. So I would define “weight shift” as “forward axis movement.”
>
> It is obvious that a “forward axis movement” does take place in the stride of many hitters. Where I run into a problem is with those (Chris Yeager for one) that maintain that this momentum is used to rotate the back-side around a posted lead-side – like a door swinging on hinges. If the back-side did in fact rotate around the lead-shoulder - like a door swinging on hinges – then the spine would also be rotating forward.
>
> Since the average shoulder width is about 20 inches, the spine, neck and head would need to rotate forward about 10 inches from initiation to contact – if the body rotated around a posted lead-side – like a door swinging on hinges. However, in all my studies, forward “weight shift” ceases (linear momentum approaches zero) before rotation begins. The spine, neck and head remain fairly stationary while the shoulders rotate more evenly around them – more like a “revolving door.”
>
> If some one can show clips of good hitters that rotate their head 10 inches forward during rotation (not the stride), I will retract my objections to the - like a door swinging on hinges – theory.
>
> Jack Mankin


**************************************************************

Jack,

Quotes from your post:

>>>“As I pointed out above, for a “weight shift” to occur that produces linear momentum, the center of mass (the spine) must attain velocity.”<<<

A definition of center of mass includes center of gravity. In fact, most of the time the two are used interchangeably, with physicists usually being the only group to distinguish between these terms. In physics, the center of gravity of an object is the average location of its weight. In a uniform gravitational field, it coincides with the object's center of mass. A good definition is : (geometric) point around which every particle of a body's mass is equally distributed. A body behaves as if its entire mass acts or is acted upon at its center of gravity. All definitions that I have seen on the subject refer to a “POINT”. The “spine” is not a “point”. To illustrate: if you were standing and bent over at the hips so that your spine was horizontal with the ground where would your center of gravity (center of mass) be? Your body has a total mass, but we are concerned with the “CENTER” of it. Center implies (and is) a “POINT”. And if you disagree, I would ask you to please look up the definition first.
So if when you say that for a weight shift to occur that produces linear momentum, the center of mass (center of gravity) must attain velocity, I would agree only if you mean the body’s center of gravity of whole body - a point ( COG located at sacral promontory anterior to S2, at 55% of body height).

Please see:

http://mysite.verizon.net/vzep5xd2/baseballphotosandvideos/id3.html




>>>“Where I run into a problem is with those (Chris Yeager for one) that maintain that this momentum is used to rotate the back-side around a posted lead-side – like a door swinging on hinges.”<<<

If that is what he believes, then I run into a problem as well. Can you site where he says/writes that?




>>>“If the back-side did in fact rotate around the lead-shoulder - like a door swinging on hinges – then the spine would also be rotating forward.”<<<

Did he really SAY that the back-side rotates around the LEAD SHOULDER?




>>>“Since the average shoulder width is about 20 inches, the spine, neck and head would need to rotate forward about 10 inches from initiation to contact – if the body rotated around a posted lead-side – like a door swinging on hinges.”<<<

An absurd argument, because I really can’t believe that anyone could think that the body’s front side (from toe, thru knee, thru hip, thru shoulder) acts as a straight pin in a hinge. Show me one batter that exhibits this action. If none perform this action, then how can anyone possibly say or believe that this is what happens?
Someone who subscribes to the “gate swing”, surely can’t mean this. They must have another interpretation. And if that is the case, then you are misinterpreting, again.
Please document where you have read this?





>>>“If some one can show clips of good hitters that rotate their head 10 inches forward during rotation (not the stride), I will retract my objections to the - like a door swinging on hinges – theory.”<<<

And I will retract my objection to your illogical argument, if you can find someone (of reputation) that subscribes to the “door swinging on hinges” theory, the way that you have described.






Please do not answer for me, thinking that I subscribe to this theory (swinging gate). And I also do not subscribe to your “revolving door”. Simple models should not be used to example complicated body movements. There is the ever present danger of misinterpretation.