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Question of the Week / Re: QotW - 11.05.29 - At what speed does windchill give way to frictional heating?

« on: 20/12/2011 12:47:38 »

May I add an alternative to racing down the highway at 200km/h in a cabrio, naked and covered in vaseline, ie. Get a scuba tank(fulll) and attach a hose to the valve.  Hold the hose opening close to your forearm, and open the valve slowly. Stop when the skin starts peeling off.

Should you feel heat at any moment during the experiment, measure the airspeed through the hose with a wind meter. Ok this idea is possibly not all that accurate, but it may give you an idea on which you can deviate.

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Physics, Astronomy & Cosmology / Re: Why does the tone change when I tap my coffee cup?

« on: 20/12/2011 12:33:26 »

I think I have finally come up with the reason why the tone changes whilst tapping:

I noticed that this only happened with frothy coffee from a expresso machine.  The liquid was well aerated firstly and as it settled, ie. the tiny bubbles escaped the liquid, the tone/pitch changed whilst tapping.  When agitated again by means of stiring with a spoon, thus aerating the liquid again, the same occured untill the air had escaped and a no tone change was noticed.

The tapping of the spoon has possibly got nothing to do with the separation of the bubbles from the liquid. It simply causes noise with which one can detect the change in density of the liquid.

Try it out for yourself..and compare it with a cup of water or tea (no change in tone).

Thanks for your time and comments, gents.  Your links are also interesting and have taught me a thing or two.
Regards, Stephen Brooke.

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Physics, Astronomy & Cosmology / Why does the tone change when I tap my coffee cup?

« on: 02/12/2011 10:26:16 »

Gentlemen, thanks for the responses.
I've tried the 'experiment'again with plain hot WATER in the following containers: Porcelain cup, single walled glass cup, double walled stainless steel cup but with NO distinguishable change in tone.
I always tap the bottom of the vessel on exactly the same spot, frequency and force.

But, when I again tried hot coffee in a porcelain cup whilst holding it by the handle and not resting it on any surface the tone changed from low to high. Here's my theory:
Due to the soundwaves and vibrations in the suspension during the tapping, the particles start separating (heavy and light) and as a result, the density at the bottom of the cup increases whilst decreasing in the upper regions of the suspension. As a result, the frequency of the vibrating cup changes along with the change in density.
Do you think that I'm on the right track? I value your objective opinions, so don't hesitate to tell me that I'm way off the mark here.
Thanks, in anticipation.
Stephen.