Re: Why would a glass of pepsi spin smoothly on ceramic and not water?

Greetings:

Your observations of spinning water and softdrink filled glasses are 
interesting. I could not find any data on the difference in volume and 
weight between carbonated and flat drinks so I tried several experiments to 
measure these parameters and to duplicate your results.

 First, I measured the volume change between flat and carbonated Sprite and 
Pepsi and water in case there might be a significant change in volume and or 
weight. I found that the volume of flat Sprite was only 3% less then the 
freshly opened carbonated drink and that Pepsi had only a 2% volume 
reduction when flat. The weight loss from the freshly opened carbonated 
Pepsi drink to the flat drink was only 0.7%. From this I conclude that there 
is not a significant change in weight and a very small volume difference 
between flat or carbonated soft drinks and flat water. 

 I could not find a crystal glass with a slightly rounded bottom so I used 
one half of a thin polystyrene sphere of the type that are available in 
craft stores to make Christmas ornaments etc. I could then set many 
different types of glass containers, including heavy crystal glasses, in the 
half sphere and spin them like a top. I could find no difference in 
stability or instability between spinning containers filled with various 
levels of water and flat or carbonated Sprite or Pepsi. What I did find is 
that the amount of water or soft drink placed in the spinning glasses did 
effect stability, whatever type of liquid was enclosed. I also tried ice 
cold and room temperature liquids and in all cases it was the way that the 
glass was spun and the height of the liquid in the container that determined 
the stability of the spinning volume, not the temperature.

From my experiments I can conclude that it is not the carbonation, the 
temperature or the type of water based drink that causes instability in 
spinning. In my experiments the physical geometry, the mass of the glass and 
speed of rotation are what effected the stability of the spinning container 
and fluid. 

I believe that the only difference between my experiments and yours must be 
related to the structure of your glasses relative to mine. Many odd things 
can happen when masses with odd mass distributions are spun.  In these cases 
it is the speed of rotation that determines stability. Perhaps you can do 
some experiments to determine where the center of mass of your glasses are 
located. If the center of mass is not located exactly on the vertical axes 
of symmetry of your glasses (the spin axis), many odd things can happen when 
spinning them.  It is difficult to believe that a few percent of volume 
difference and less than 1% of weight difference between carbonated and flat 
water are key stability factors. The problem is that you indicated that the 
effect that you observed was independent of the liquid level in the glasses 
making the variation between carbonated and flat water to be an 
insignificant factor.

Still scratching my head about this question, Your Mad Scientist
Adrian Popa