Re: Why do cans of diet soda turn horizontally when they float?

The question: "Why do cans of diet soda turn horizontally when they float? I understand why cans of diet soda float and cans of regular soda sink. But why do the diet soda cans float horizontally, while the regular soda cans stand upright after they sink?"

Previous information that you may have seen:

• A cool movie
• previous answer at MadSci
• another previous answer at MadSci

Now, as to the specific question about why the cans that do float do so horizontally: It is one of the stable orientations. But you may have noticed in the movie above that, at least for the length of time of the movie, the Diet Coke can is apparently stable in its vertical orientation! Here are a couple of articles at Wikipedia that have some useful information:

• mechanical equilibrium
• saddle points

In fact, I'm not so sure that the horizontal position is as stable as the vertical position, because in the horizontal position any deviation from perfectly horizontal may provide enough torque ("turning force") to cause the can to flip to the vertical position as the contents of the can move around inside. It is a matter of how far below the physical center of the can the center of gravity is in the two positions, and how much, physically, the can must be disturbed in order to cause the contents to shift around for sufficient torque to occur.

You may want to do some experimenting to see how much disturbing of the can is necessary to get it to change from the vertical orientation to the horizontal, and back again. Have fun!

John Link, MadSci Physicist