Here’s your chance to try to outsmart the vasy majority of the Stony Brook physics department (including a Dirac Medalist, several Guggenheim fellows, and so on). At our colloquium on Tuesday, a speaker from UMass-Boston, Dr. Arthur Eisenkraft, gave a talk about physics education strategies, and along the way posed the following problem to the audience: consider a pendulum which consists of a large bathtub (filled with water) swinging back and forth. Now imagine the pendulum is positioned next to a faucet, such that every time the tub swings back to the right, it comes under the faucet and a little water is added to the tub before it swings away:

Ignore the fact that the force of the water pouring in will disturb the oscillations a little bit, and the fact that the water will slosh back and forth. Imagine perhaps it’s a very thick liquid like honey instead so that it doesn’t move much while it is in the tub: the point is that the volume of fluid in the tub (and therefore the mass of the pendulum) is gradually increasing a little bit with each swing.*

The question is, as time goes on, does the pendulum speed up, slow down, or stay the same?

Dr. Eisenkraft asked us to vote on what we thought the answer was by show of hands, and I would say about 70% of those of us in the audience (self included!) turned out to have gotten it wrong. Granted, we were working on the fly and with slightly more time to think about it, I’m sure some of them would have worked out the right answer (and I’m pleased to say, two of the three fellows who I think of as potential thesis advisors got it right right off the bat!). Nevertheless, it’s a bit harder than it looks. Make your guess, then check the answer below the fold:

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