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Whilst studying for my physics courses, I like to drink tea. Today, I noticed that if you pull a bag of tea out of a hot cup of water, it gradually starts to rotate, picking up speed as time progresses and reaching a asymptotic speed after a while. My first question is: why does it start rotating? Of course it has something to do with the liquid water entering the gas phase just above the cup of tea, but how does that give the bag of tea a directed motion to one side? I presume that the momentum of the gaseous water is quite randomly directed vertically, so I don't see a reason for a horizontal rotation. Second question, how is the direction determined? I haven't tried, but might the direction of rotation be equally probable to be left-handed and right-handed? Third question: might the shape of a tea bag have an influence on what we observe? If we take a pyramid tea bag, for example, does the behavior change? I will perform some of these experiments while making my cup of teas and report back to you :)

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I think it's just the torque that's stored in the thread... –  Juris May 18 '13 at 12:35
    
@Juris, I thought that as well, maybe the water makes the thread more flexibel, llowing it to unwind more easily. –  fibonatic May 18 '13 at 12:51
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up vote 11 down vote accepted

Of course it has something to do with the liquid water entering the gas phase just above the cup of tea, but how does that give the bag of tea a directed motion to one side?

Nope.

The teabag is dangled by a string. Remember that the string is made of wound up threads:

enter image description here

Now, the threads stay wound up because they fit well and they have a knack of being permanently deformed if held in the same orientation for too long (for example, if you loop a thread and press it for a while, the loop stays). Wetting them undoes this permanent deformation.

When you dip it in tea, however, the liquid is absorbed by the threads, and they expand. There is no longer enough room for them to stay tightly wound. Besides this, they lose the permanent deformation, so they have the motive and the means to unwind -- which is what happens.

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Unexpected but beautiful answer! So if we dip the tea bag into the water, without wetting the thread, it will not rotate? I will try it! –  Funzies May 18 '13 at 13:01
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@Erik: Well, depends, because water way rise up the bag and into the thread. (Plus the steam may be hot enough to loosen the thread). But the general idea is correct :) –  Manishearth May 18 '13 at 13:02
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