more on the grip …. 2

Evidence #2: frequencies of free, clamped, and hand-held bats

A more reliable experiment was carried out in 1990 by tennis science expert, Howard Brody included data for hand-held baseball bats. He compared the vibrational behavior of wood and aluminum bats which were free at both ends, clamped at the handle end, and hand-held. He mounted a strain gauge on a wood bat and struck it with a baseball and recorded the resulting vibration response of the strain gauge on an oscilloscope. He found that the clamped bat exhibited what is called a “diving board” mode at a very low frequency of 18 Hz, as well as a higher frequency, second mode at 209 Hz. The free bat, however, did not exhibit this diving board mode at all, but instead its lowest vibrational mode was similar to a free-free beam, and had a frequency of 163 Hz. He found different frequencies, but the same trends for an aluminum bat. The animations below illustrate what the first two diving board and the first free-free mode shapes look like. The relative speeds with which the aminations is similar to the ratio of frequencies Brody measured.


1st diving board mode for a clamped beam

2nd diving board mode for a clamped beam

1st bending mode for a free-free beam

When Brody measured the vibration of hand-held bats (both wood and aluminum) he found absolutely no evidence of the low frequency diving board vibration. For both tight and loose hand grips he found only a higher frequency vibration, very close to frequency of the the free-free bending mode. However, this vibration was severely damped by the hands and died away very quickly, with a tight grip damping the vibration more quickly than a loose grip. His conclusion was that “a freely suspended bat corresponds to both the loosely held and very tightly gripped bat, and a bat firmly clamped in a vise does not behave the way a hand-held bat does.” One implication of this conclusion is that a player could essentially throw the bat at the ball and it would have the same effect as gripping the bat tightly in an attempt to get more power out of the collision with the ball.

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