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Bizarre Observation

JustCallMeTarzan
Posts: 1,922
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10/7/2011 4:55:54 PM
Posted: 5 years ago
So I'm no scientist, and this may actually be completely normal.....

But when I microwave a cup of coffee, add my cream and sugar, then stir... I usually take the spoon out while it's still swirling and tap it on the edge of the cup to knock off any drips. But curiously, as the coffee swirl slows, the pitch of the sound the spoon makes goes UP, not DOWN.

One would think the faster the coffee is spinning and the hotter it is, the higher the pitch would be, no?
Lasagna
Posts: 2,440
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10/8/2011 9:35:11 AM
Posted: 5 years ago
The swirling motion is complicating your observation - all that is being measured is the heat, which is rapidly cooling as soon as the coffee comes out (which is causing the sound change). I'm not sure exactly why, but when I have a really hot liquid in a cup, the structural integrity of the cup seems to diminish while it is in the hottest state. I often do a similar thing: I hit the spoon on the bottom of the cup (while it's inside the liquid) and wait for the tings to raise in pitch until they nearly disappear. When the liquid is at it's hottest, it's much easier to break the cup. The hot liquid makes the ceramic/glass weaker, and the low-pitched tings are representative of the fact that you are very nearly ready to bust it, which makes some sense.
Rob
RoyLatham
Posts: 4,488
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10/16/2011 12:11:05 PM
Posted: 5 years ago
I see a major book on coffee cup physics ahead.

My thinking is that because the hot air is less dense, the spoon vibrates at a higher frequency. This is the same effect as using helium to get a high pitched voice. The vocal chords vibrate faster in the low-density helium.

The speed of sound is faster in hot air than cold, but off hand I don't see how that would change the pitch of a tone already generated at a given frequency. It must have been generated at a higher frequency.

A drop of milk in coffee can be used to demonstrate an important aerodynamic effect. Holding the spoon at a low angle with respect to the direction of motion, move the spoon in a straight line. This simulates an airplane wing. The drop will swirl around and be cast downward. That is the wake vortex shed by the "wing." The trailing wake vortex from a large airplane can flip over a small plane. Pilots keep separation from large airplanes to avoid the problem.
JustCallMeTarzan
Posts: 1,922
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10/16/2011 12:30:22 PM
Posted: 5 years ago
Well I've made some additional observations that continue to baffle me...

First, hot water does not exhibit the same effect, so it can't have to do with the swirling motion changing the size of the echoing tube or the structural integrity of the cup.

Second, the pitch goes DOWN when I'm dissolving the sugar.

Third, even after the sugar is completely dissolved, and the surface is still, tapping the spoon on the side makes the pitch go UP - considerably more than an octave.