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Re: Jeff -- Rotation /Momentum


Posted by: Jack Mankin (MrBatspeed@aol.com) on Thu Dec 23 00:04:06 1999


Hi Jeff

The concept that weight shift contributes in any significant degree to hip rotation in incorrect. The linear momentum of the hips developed during the stride is NOT converted into angular momentum. I know that on another site (Setpro.Com) Max and Adair have put forth theories that states there is this conversion. The analogy they use to prove their point is that of a runner tripping. The feet stop but the upper body arcs forward and now has angular momentum. The analogy is that once the front hip stops, momentum would cause the back hip to arc around it. I find this analogy very misleading.

Max has the body moving forward (vertically) at 5MPH. Once the feet stop, he has the head instantaneously attaining 7.5 MPH. I can only wonder what that rate of acceleration would work out to be. But, for the sake of the discussion let us stipulate it to be true. For this analogy to pertain to the baseball swing, the hips would have to be fully open (hips perpendicular to direction of movement- like the runner) and each hip moving forward at 5MPH when the front (I guess we now mean the left one) hip stops. Of coarse this is not true. When the front leg plants, the hips have not rotated and the hip joints are still inline (linear) to the direction of movement.

In an earlier post, I asked Jeff: “Why doesn’t an arrow snap off when it strikes a tree.” The answer to that question is the same as why the linear momentum of the hips is not converted into hip rotation. --- The arrow does not snap because the momentum is directed down the length of the shaft and is absorbed into the tree. If any of it’s linear momentum were converted into angular momentum it would have snapped the arrow.--- The linear momentum of the hips is absorbed into the lead leg and not converted into hip rotation

Jack Mankin


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