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[Phys-l] Amplitude from speed at BDC Was: Re: real-world mechanics problem



Top post

Actually no data required to answer my question tho as a practical prob. (AKA a non-imaginary world prob.) one would need data to find various effects from the quarters. I thought I'd supplied enuf. Later. My question was why are the speeds at BDC different for the two methods of derivation? [kinematic and total energy]

Now mid post(s)

On 2011, Feb 03, , at 12:52, John Mallinckrodt wrote:


Beyond the fact that I can't find any data with enough information to make any use of, it seems to me that you are barking up the wrong tree to try to relate the period change to an amplitude change that depends on where in the cycle you add the quarters. (If I read correctly between the lines, you seem to be treating this as an inelastic collision between the pendulum and initially motionless quarters with an energy loss occurring if the pendulum happens to be moving.)

No, and partially yes, I think. The quarters can be added when the p is either not moving or at BDC, which is when dropped vertically will not affect the horizontal motion. In either case they must be done carefully and as I suggested to Bryan, a number of trials to verify any effect is not due to an initial motion of the quarters.

Yes if added on the "up swing", the P doesn't rise as high because the quarters "use up" some of the KE. If added at the down swing no change.


First of all, the drive mechanism will presumably act to counter any amplitude change that might result from whatever mechanical disturbance might occur during the quarter placement.

OML, and I know of a class of escapement in which the P motion is even reversed, the amount depending on the amplitude. Not w/ a massive bob, such as this tower clock tho.




Second, the period change from adding quarters will likely have more to do with the change in the relative mass distribution (e.g. altering the radius of gyration) than any change in amplitude.


The relative change in rod length is what changes the period. I, again, stupidly, thought I could treat the P simply w/ the C of M just moving up a little. So back to the thinking cap. However, I suspect my suggestion of adding not 0.02gm (~ three quarters, but 200 would show a sig. decrease, and easily calculated, in the amplitude. I think it would be an unusual coincidence if the measured matches the calculated and is due to the radius of gyration change instead of a "sopping up" of the KE.


Anyway, I'm Bccing this to Bryan I hope he'll disconnect the escapement and do some trials. He did this for me on his Synchronome, but he owned the Sync. He doesn't own the tower clock.


bc Die Panzer to JM for pointing out the too obvious and a subtle one.

p.s. I wrote leapfrogs with a varying rod length and g, but don't remember the result (five years ago?) and have lost my pendulum journal, merde!

p.p.s. I think all we need to know for calcs. (first quote) is rod length, bob mass, quarters' mass, and amplitude. (simple p) For the physical, the dimensions of the bob (a cylinder) and the rod length (suspension spring upper chops (clamp) and rod to the top of the bob. Is the rod attached in the depth of the bob w/ a different metal sleeve for temperature compensation?

bob mass: 77kg
simple pendulum equivalent rod length: 1.55 meters
typical amplitude 50mR (v. ~ 2.9 deg.)



John Mallinckrodt
Cal Poly Pomona