# Centrifugal coffee cups

1. Oct 25, 2004

Heres something interesting I noticed the other day.
I had just made a cup of coffee in a ceramic mug,
and had just finished stirring it. Absent mindedly,
I started picking up my spoon and dropping it into
the cup at roughly half second intervals.

On doing this, I noticed something very peculiar
that baffled me at first. Each time the spoon hit
the bottom of the mug, it rang with a frequency
that was noticably higher than the time just before.
I kept this up for a while, experimenting by
dropping the spoon from different heights, I tried
increasing and decresing the time intervals between
spoon drops, but none of these things changed the fact
that the frequency increased steadily each time the spoon
dropped.

After a while, I noticed that the coffee in the mug was
still swirling, but very slowly. I stirred it rigorously,
and this time the frequency was low and once again increased

Does anyone have any ideas as to why this happens?

Here was my initial thought:
The swirling coffee exerts a centrifugal force on the
walls of the mug, as the swirling slows due to friction,
this force decreases. Could it be that this centrifugal force
exerts enough strain of the walls of the mug to alter the
pitch of the sound produced to the extent, that the change in
frequency can be clearly heard after just half a seconds change
to the centrifugal force.

The reason Im asking is that as Im sure you can imagine, this
force must be miniscule, and even when the coffee was hardly
swirling at all, the change in frequency was still easilly
discernable.

Im curious to hear any other possible explanations you people
might be able to think up.

Last edited: Oct 25, 2004
2. Oct 25, 2004

### Tide

My understanding of the phenomenon is that pouring hot liquid into the cup sets up substantial stresses in the ceramic. By "tapping" on the bottom you are relieving some of that stress and so the pitch changes as you relieve more and more of it by tapping.

When you leave the system idle for an interval, the liquid and cup exchange thermal energy and tend to reduce temperature gradients. By agitating the system again you reintroduce higher temperature gradients and, again, greater stress.

3. Oct 25, 2004

Thanks a lot... that sounds a lot more feasible.

4. Oct 25, 2004

### niehls

i also encountered this phenomenon for the first time about a year ago. I had almost forgot about it until you reminded me. (thanks by the way). I don't really understand the explanation tide supplied us with. initially i was thinking about the curvature in the surface of the coffe and that it might alter the wavelength of the sound wave in the cup. But as the center of the surface goes upward (when the swirling in the coffe diminishes) the wavelength should get longer, and hence the frequency should go down (If you don't consider the edges of the mug). Could you please explain a a little bit better tide? I haven't read much physics (1 year at university) so i might find the explanation hard to understand though...

5. Oct 25, 2004

### Tide

niehls,

Actually, I was recalling what a materials physicist related to me years ago. I hadn't thought much about it and so I decided to do a Google search on the topic. Apparently, the differential stress solution of the problem has been discounted in favor of variable sound speed in the liquid due to air being trapped in it.

Here's one discussion I found: http://www.newscientist.com/lastword/article.jsp?id=lw280

Last edited by a moderator: Apr 21, 2017
6. Oct 26, 2004

### krab

If you pour hot water from the tap directly into a clear glass, it's a lot easier to see what's happening. There is a 'mist' of fine bubbles that is uniformly distributed throughout the liquid when you stir it. When you stop stirring, the bubbles can rise, gradually clearing from the bottom up. The effect is due to the fact that the speed of sound is different with the bubbles mixed in than without.

BTW, if you do this experiment, don't let the water blast through the screen (atomizer) that's usually mounted at the end of a typical kitchen tap. That takes all the microbubbles out.

7. Oct 26, 2004

### niehls

thanks tide, that link was really helpful. I recall that instant capuccino or latte works alot better than plain coffee. Is this because more bubbles are caught in the liquid? I think the way to make this phenomenon most noticeable is to start yourself a nice fast swirl and then stop and start tapping the bottom of the cup. as the swirling stops, the tone pitches up. I'm going to have to dig deeper in this little problem though.
I've only been active in this forum for a couple of days and i already love it. I'll probably spend alot of time here for the rest of my years as a physics student :)
cheers from sweden

8. Oct 26, 2004

A fascinating thread.... and what a great excuse for a coffee break!

9. Oct 26, 2004

### Integral

Staff Emeritus
Humm... I just tried the experiment, I did not observe a significant effect...BUT....

I don't pollute my coffee with sugar and fat. Black is the only way... also I brewed the pot about 1 hr ago. Most likely it has very little dissolved air, even vigorous stirring does not add much. Most likely when you add a powder, either creamer or sweetener it carries a LOT of air which is mixed into the coffee. Thus the effect will be strongest when a finely divided solid is added.

10. Oct 27, 2004

### Chi Meson

This effect works best with cocoa. THis supports Integral's idea.

But I still like Tide's explanation best. The pitch "resets" to a low tone innumerable times after waiting ten seconds or so, and then rises with each tap of the spoon. "Bubble Theory" will have to explain how these bubbles come and go!

Different kinds of mugs make a huge difference in the effect. Ceramics are strange and wonderful. I have a strong compulsion to want to believe the "temperature gradiant" idea.

Edit:
I also recall that tapping more quickly quickens the rate rise of pitch, and I want to stress that no stirring is necessary to make the pitch fall again. I will not sleep until I understand this! (I'll be drinking too much coffee)

Last edited: Oct 27, 2004
11. Oct 27, 2004

### airbuzz

Hey men, we could win an IgNobel from the improbable research annals if we do a deeper study on this!!! Let´s think about it :surprised
If you don´t know improbable research go here
Lot of funny and intelligent stuff

12. Oct 28, 2004

### Chi Meson

RE my last post:

I am now a "bubble" theorist. I made 5 mugs of cocoa, and can now say that my previous perceived obsevations were not borne out. Looks like I so much wanted this to be due to the structure of the mug that I allowed my prejudices to warp the data. I am ashamed
I turn in my slide-rule.

13. Oct 28, 2004

### ZapperZ

Staff Emeritus
What is a "slide-rule"?

:)

<runs and hides - a very common thing for me to do nowadays, it seems>

Zz.

14. Oct 28, 2004

### Integral

Staff Emeritus
LOL!

Kids these days! Hate to admit it but I even learned to dirve one of those things. That was about 5 BC (Before Calculators)

15. Oct 28, 2004

### Chi Meson

Same thing as a log-table.

16. Oct 28, 2004

### ZapperZ

Staff Emeritus
Log-table.... is this something we find in a log-house?

<Runs screaming with fear as dozens of abacus being thrown in his direction>

:)

Zz.

17. Oct 28, 2004

### LURCH

Think I tend to agree more with the explanation involving the shape of the fluid's surface. It seems to me that one can achieve the same effect with a cold liquid, or one at room temperature. However, if memory serves, the effect is much more noticeable in a taller and narrower glass.

18. Oct 28, 2004

### tomkeus

I only think that you should start to worry about the structural integrity of your cup.

19. Oct 28, 2004

### Phymath

watch apollo 13 when tom hanks asks about his calculations all the flight engeinerds use slide rules...

20. Oct 28, 2004

### Integral

Staff Emeritus
Slide rules had some very nice features.
No batteries must top the list!
They were fast to get results in complex operations which involve many keystrokes on a calculator.
At best you got 4 significant digits (usually 3). This is a big one in my mind, considering how frequently I see people spew the entire contends of their calculator display into the forums as if it had some meaning.

The biggest draw back was the inability to do simple addition and subtraction. The trade off was the near instant acess to trig functions and logs.