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Author Topic: Kitchen science: Stirring liquid in a cup causes the pitch to change when you tap the cup.  (Read 3490 times)

Michael Finley

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Michael Finley  asked the Naked Scientists:
   Dear Chris,

I listen to you over podcast from Atlanta, GA. 

I've noticed something that would be a good kitchen science experiment, as it has an audio component.

When I take a large cup and fill it with something to drink, like tea or coffee, I usually stir it up. As I am stirring, each time the spoon goes around the cup, it taps the side. The pitch of the sound made by the spoon goes lower and lower each time around as I stir. Once I stop stirring, I can continue to tap the side every second or so (again, with the spoon inside the cup) , and the pitch goes back up again.

Great show,
Mike Finley
What do you think?


 

Offline LeeE

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As you stir the fluid in the cup the fluid rises up the insides (the fluid is being thrown out by the stirring but is constrained by the walls of the cup, so it can only rise up the walls) and this will change the pitch as it changes the length of the resonant cylinder.  However, I'd expect the pitch to rise as you stir instead of dropping.  Then again though, as the fluid rises up the insides of the cup the fluid at the center will drop, lengthening air column, so perhaps that's why the pitch drops.

Dunno for sure - I'll have to try it sometime.
 

lyner

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I thought there might be some 'loading' of the walls of the cylinder by the water on it. That could lower the resonance. It could be a different situation from just having deeper water - which, as we know, raises the frequency as it puts a node at the horizontal water  surface (there's significantly more mass with all the water and it will reflect the wave rather than just loading it).
 

Offline fragglerock

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I have not come across the effect in tea, but definitely with instant coffee it is very marked.
I understood it was to do with the tiny air bubbles released from the coffee granules affecting the way the water vibrates, and as the bubbles rise to the surface and break the note changes.

Hmm seems like I probably got that info from new scientist.
newbielink:http://www.newscientist.com/article/mg15120499.000-the-last-word.html?page=1 [nonactive]
newbielink:http://www.newscientist.com/article/mg15120499.000-the-last-word.html?page=2 [nonactive]
 

lyner

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Do you actually need bubbles for this to happen? You can get the effect with water sometimes.
 

Offline LeeE

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I like the loading of the walls answer.  Wouldn't this be akin to raising their effective mass?
 

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