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Weight-shift / Rotation

Posted by: Jack Mankin (Mrbatspeed@aol.com) on Thu May 10 09:15:23 2001

Hi All

A business trip has kept me away from the computer for a few days. Since returning I have read most of the e-mails and new posts to the board and will try to catch up as soon as time will allow.

Over the past couple of weeks there has been a lot of discussion regarding linear weight-shift and its relationship to developing body rotation around a stationary axis. There have been many examples given of how a linear motion can be diverted to add to an athletic movement such as high-jumping or similar activities. Diverting a linear motion into a different direction (or vector) is well covered by the laws of conservation of momentum, and of course, I would have no objection to them. But I do have a problem with the explanation given for how linear hip momentum is transformed into rotation around a stationary axis (the spine).

This certainly is not the first time I have debated this subject or those similar to it. For those readers who go back a few years, you will probably recall that Paul and Mad Max from Setpro, Jeff, Steve T and others have discussed this very topic in some detail. I am sure Paul remembers our discussions and drawings of "The runner that tripped." --- I think it might be a good idea to revisit that discussion as a point of reference. It certainly has more validity than the argument currently being presented.

One of my first disagreements with the theory that converts linear momentum into bat speed was the "weight-shift and extension" theory. The theory basically contended if a batter shifted his weight forward and extended the bat lengthwise (knob first), and when the hands slowed, the bat-head will come flying through like "the crack of a whip." --- For those of you that have my video, you saw the results of the test that disproved that contention. This theory is also discussed in {Test the "Crack of The Whip" Theory} from the site map.

When the hands came to a stop after the batter shifted his weight forward and accelerated the bat (knob first), the bat-head came to rest with no angular movement. The barrel was still pointing at the catcher. The laws governing the conservation of momentum that kept the bat-head from rotating in that test are the same laws that keep linear hip movement from being transformed into hip rotation. --- In the case of the bat, the linear momentum vector is directed "in-line" ("n" factor = 0) down the bat's length and was absorbed in the hands. In the same way, the momentum vector of the hips is directed into (and absorbed by) the lead leg. --- Shift your hips into a wall and see if there is any tendency for them to rotate.

We can now revisit Paul's "runner that tripped" analogy. --- Here we have both feet of a runner coming to a sudden stop after striking a solid object. Would not the linear momentum of the runner"s body be converted into angular displacement as the torso arced downward? -- Yes, this is in agreement with the conservation of momentum laws. But here, we are not speaking of linear motion that has in-line vectors as was the case with the bat or hip-slide. The linear vectors of the runner's feet and head are "in-parallel." The batter would need to stride with the hips fully open (front and back hip moving parallel with each other) for the runner analogy to be a valid comparison.

As I said earlier, Paul's position does have some validity. In that, if the batter should stride with the hips (say) 20% open, then about 20% of the hip's linear momentum could be converted into hip rotation. --- It is important to note that in this case the center of rotation is not the center of the body. In this case, rotation is around the lead-hip. --- The argument now being presented has the momentum of a linear hip-slide being converted into rotation around a stationary axis (the spine). The laws governing the conservation of momentum will not support that argument. I will give my reasons why this is true in another post.

Jack Mankin


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