This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment Well, I didn't mean to imply that the impulse at the pivot is all = absorbed; but it is all lost as far as the string is concerned. The center of percussion is one of those counter-intuitive phenomena in = dynamics. For the multitude of college students that only take a = semester or two of physics, the concept of forces on a free body acting = at the center of gravity is a paradigm that is hard to dislodge. For an = object in free space or translating in a straight line, which comprises = most situations, the center of gravity is always used. But for an = object pivoted at some location other than its c.g., all bets are off. = The most publicized version of this is a baseball bat's "sweet spot" as = you described. Think of a wooden board hung from a pivot point at its top. If you = strike the board up near the hinge, the pin will experience a force = coming from the direction of the strike. But if you strike the board at = the bottom, the pin will experience a force in the opposite direction as = the board tries to spin about its center. There is a point in between = these two where the pin force is neither positive nor negative and the = pin will actually see no force. This is the center of percussion and is = not located at the centroid of the board. It's not usually even = particularly close. It would seem to be a good idea to design the hammer so that the string = contact point of the head passes through the center of percussion. That = way the maximum amount of energy would be transmitted to the = string...like dropping a ball on it. But I'm not sure if they actually = design them that way. Don A. Gilmore Mechanical Engineer Kansas City ----- Original Message -----=20 From: Sarah Fox=20 To: Pianotech=20 Sent: Thursday, December 18, 2003 2:38 AM Subject: Cockeyed hammers / Don Gilmore Probably any baseball player would know that it's not smart to hit the = ball with the very tip of the bat. It makes for a nasty shock to the = hands (and to the extent that the hands are not rigid in space, it takes = away from the power transferred to the ball). Why do they do this with = piano hammers? More to the point, is this not considered an = unneccessary demand to put on the hammer flanges and bushings? Perhaps = it's also a drain on efficiency? Wouldn't it make sense to have some = sort of asymmetrical molding whose center of mass is on the far side of = the action center, so as to put the center of mass of the hammer = assembly closer to the line of strike? Also, wouldn't it at least make sense to angle the head of the hammer = inwards slightly, so that the radius to the head's center of mass is = perpendicular to the strike axis of the head? It seems to me that this = would help to control wobble in the hammer I'm not sure I agree about part of the impact being "absorbed" by the = pivot. Sure, there would be force against the pivot, but for impact to = be absorbed, the pivot would need to be compliant and inelastic. I = realize that's true to an extent, as no system is ideal. However, do = you think this would be a substantial drain of energy? It seems to me = that the hammer felt and the strings are far more compliant and are = closer to the center of mass, such that almost all of the energy would = be dissipated there (rather than at the comparatively rigid center). = Have you ever measured this? Just curious on your take. I can't do much more than scratch my head = about this one. If the hammer were perfectly rigid and the center = bullet proof and noncompliant, there'd be nothing to worry about. But = considering that it does the hokey pokey and shimmies all about... = Well, I don't know. Peace, Sarah ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/9a/c2/71/05/attachment.htm ---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC