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Re: [Phys-l] Student Misconceptions



Chuck's response looks reasonable to me - but it reminds me that I watched the Mythbusters scale up a Newton's Cradle arrangement, with various sizes of steel balls, last night. All seemed to go well up to the six inch - 33 lb ball size, so echoing a YouTube video (the source of several of their endeavors) where cranes lofting a row of five wrecking balls was seen to behave plausibly. It was at the full scale replication that things went badly wrong for the fearless investigators. Their measure of merit was the half amplitude loss, between the first ball's initial deflection, and the last ball's half amplitude, expressed as a % loss of initial amplitude. The smaller sizes showed this figure of merit to decline slowly with increasing size, from a 2% loss, then a 3% loss, then a 6% loss - but as the one ton size was approached, the loss exceeded 30% so that the end to end swings declined in number to just two.They concluded their concrete filled balls were lossy, so they reworked the one ton size with an equatorial steel disk capped both sides, and filled with concrete on both caps. Still no joy. Then double suspensions like the smaller balls had. No joy again!

Nobody mentioned the curious clearly visible route that the initial potential energy was heading: As the first ball descended, the remaining balls started to move in the same direction. The same feature that provides pendulum clocks with an important loss path: the suspension support structure.

Brian W

On 10/7/2011 7:53 AM, chuck britton wrote:
I'll take a stab at this.

If the mass miraculously increases at the instant that the bob is at
it's lowest point, then the amplitude will decrease.

If the mass miraculously increases when the bob is at it's highest
point, then the amplitude will increase.

Am I missing something? (except for maybe someone else's response -
further down the mailbox?)
.
At 12:08 PM -0700 10/5/11, Bernard Cleyet wrote:
Aaa, perhaps Alphonsus' misreading of my request is a clue as to why
I've not received a response.

My request was (is) what happens to the amplitude of a pendulum
clock when the bob's mass is increased (without changing the CoM).

Not more misconceptions.

bc frustrated.

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