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Published: 20 December 2019

• physics
• fluids
• mechanics

This is a simple demonstration, that students can easily try at home. It can also be valuable in class, and is recommended for introductory mechanics lectures. It is perhaps most suited to the second day of discussing center of mass and balance, after students have been introduced to the absic concept of what center of mass is and how it affects the equilibrium position of an object under gravity.

An empty soda can can sit upright on its bottom, or can be laid on its side, but cannot be at rest at any angle between these. However, this can be changed by adding a liquid to the system.

Pour approximately 150ml of water into the can, and then try carefully balancing the can at an angle, as seen in the photo above. (This may require experimenting to find the exact right amount of water for any given can; we recommend doing this in front of the class so they can see the process.)

Ask your students why this should happen? The mass has increased, but why does that change how it balances? The water moves when the can tilts, causing the center of mass to shift – with just the right amount of water, the new center of mass will be above the edge of the can, and so it will balance.

Some cans will tend towards a particular orientation and will roll along the edge to that point; invite students to hypothesize why this is. They may see that the location of the hole and tab in the top of the can affects the equilibrium position – consider how this can be used in class to relate to the concept of symmetry. The can’s behaviour as it reaches equilibrium is a damped harmonic oscillation, and is a good introduction to how fluid action can cause damping from within a system, as well as from an external source.

As you plan for your next class, check it out on our website at B1-18: Center of Mass - Soda Can and Water.