Posted by: SteveT () on Wed Apr 5 11:07:43 2000

Hi Jack,

As usual, your logic is impeccable. The effective mass of the bat is considerably lower than that indicated by the total weight (I hate using the two terms together because mass really is NOT weight but in a given gravitational field it is.) The effective mass will not change by choking up, but the bathead speed attained may.

The theory is that since impact time is so brief, by the time the wave set up in the bat due to impact reaches the handle (hands), the ball is long gone from the bat- no time for the body mass (or grip) to influence the magnitude of impact. Think of it as pluckimg a guitar string.

But because of an old study I turned up on grip firmness vs. ball exit speed, I've begun to question part of this premise, and here's why:

Is the instantaneous amplitude of a guitar string's vibration influenced by the mechanical characteristics of the instrument's bridge? I believe to a degree it is. Interestingly, the study I refer to found no difference in velocity for various grips with a wood bat, but significant differences with aluminum. Perhaps the higher resonant frequency of metal bats has something to do with this (trampoline effect also.) I'm currently conducting tests on exit speed with and without a procut attached with batspeed constant- this may shed some light on the "vibrational reinforcement" theory.

As I've said before, welding the bat to a flywheel is not comparable to holding it in a pair of human hands. I believe the extended shape and dissimilar mechanical resonances/ impedances would not give the results you would expect anyway- the momentum at impact would not be velocity X 200 lbs. For one thing, you have to look at the tangential velocity of the flywheel- is the entire mass moving at the same speed as the bathead? No, it is not. But this does give rise to the idea of "rotational whip effect"- what happens if the bat is allowed to pivot rather than being welded to the flywheel? What happens to the bat if the flywheel is accelerated to high speed and then slowed/stopped?

The fact that torque is a large contributor to batspeed does not affect the above description in terms of body mass in a significant way as far as I can tell. The manner in which energy is transferred to the bat is irrelevant as the time is so short (less than a millisecond.)

Let me also add, however, that my background is not in classical physics/ Newtonian mechanics but in engineering and the biosciences, so I should not be considered an "authority" on the subject.

The bottom line (the way I see it)- body mass only influences batspeed; is not a factor in the momentum calculations, grip firmness is a factor with metal bats but not with wood.