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Q & A: Vibrating Coin Trick

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Most recent answer: 03/06/2011
I wasn wondering if you cna explain to me an experiment that my teacher performed in class.. I canít understand the meaning of it. She had a a empty beer bottle and then mosited it and placed a quater on top of it. Then she took the bottle and placed it in a beaker filled with warm or hot water. After she did this the quater began to vibrate and move, it was like the coin was dancing. She then was asking what just occurred. I couldnít underssttand what happen to make the coin virbrate.. I canít even unserstand how th eexperiment was related to our science ask.. I looked around for explanations but I still canít understand the reason.. I hope that you can help me with it.. Thank you..
- Ding Manibusan
University of hawaii Lab school, Honolulu, Hi
Ding -

I think that what was happening has to do with the air in the bottle. When your teacher set the coin on top of the bottle, some cool air was trapped inside. But when she put it in the hot water bath, the air inside began to heat up. Hotter molecules have more energy- they're moving around faster. That's why when you heat up a balloon, it expands- the hot air molecules are bumping into it more often and harder. So they bang on the bottom of the coin more than the cooler air molecules outside bang on the top. The pressure from those molecules below actually lifts the coin up and lets some of the hot air escape.   You can think of the hot air as taking up more space, because the molecules push out more and expand the container(balloon)  or escape it (bottle).  This is probably what made the quarter vibrate.

-Tamara (+mw)

(published on 10/22/2007)

Follow-Up #1: beaker

what is a beaker
- kevin hopkins (age 13)
calumet city il us
A beaker is just a straight-walled glass vessel, not tapering in or out,  used in a lab.

Mike W.

(published on 11/13/2007)

Follow-Up #2: The vibrating coin effect

If one conducts a vibrating coin experiment, will the length of time that the coin is left in the refrigerator affect the intensity or length of time that the coin vibrates?
- Andrea (age 12)
Hot Springs, AR, USA
Leaving the coin in the fridge won't change the effect very much.  However leaving the bottle in the fridge will affect it quite a bit.  It's because the temperature differential of the air inside the bottle will be greater when it starts to warms up in the outside air.  According to the (not exactly ideal) "Ideal gas law" the change in the pressure of a gas at constant volume is proportional to the change in temperature.   See:    When the air inside the bottle warms up the internal pressure rises.  The force on the coin due to the imbalance between the internal pressure and that of atmospheric pressure will be enough to overcome the force of gravity holding the coin down.  Eventually the coin lifts a bit, releasing some of the excess pressure.  This keeps going on until the internal temperature is almost the same as room temperature. So the time and intensity of the effect is directly related to the change in temperature.  The gas flow from the bottle as it warms will be greater if it starts out in the refrigerator because it has more warming to do. 


(published on 03/06/2011)

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