When stirring sugar into my tea, why does the pitch of the stirring sound rise?

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furniture@work  asked the Naked Scientists:

Okay, First I'd like to say that your shows are the best and that I very much enjoy listening to them. Thanks for your efforts and keep 'em coming!

I've a question for you...

First the situation:

I'm stirring a cup of hot coffee to dissolve the sugar I've just added. Whilst the liquid is still spinning round in the cup I begin tapping the bottom of the cup to see if all of the sugar is dissolved. Then I notice that the pitch of the sound produced by the spoon tapping the cup's bottom gradually increases and then levels off after a few seconds.

Question: Why does the pitch rise?

My guess is that the water molecules are packing in more densely as the stirred liquid slows...or something along these lines.

Again, props to all involved in your podcasts. They rock!

My name is Michael, 3D furniture modeller in Houston, Texas

What do you think?


Offline neilep

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The exact same thing happens when you add powdered milk substitute to coffee....I'd like to know why too !
Men are the same as women, just inside out !


Offline RD

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The denser the medium the faster sound travels through it...

Adding soluble substances will increase the density of the liquid in the cup and increase the speed of the vibrations, i.e. increase pitch.

« Last Edit: 04/07/2008 22:50:16 by RD »



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Modeller Mike thinks stir turbulence is key