This week we’re taking a look at a classic illustration of center of mass: the Brass Barbell. This simple device is a brass rod with solid brass discs on either end, one larger than the other. It can sit balanced on a stand; but, because the disc on one side is larger and thus more massive than the other, its center of mass, the point at which it balances, is not in the geometric center of the rod.

You can see this in this video with Dave Buehrle:

 As Dave explains, what we see here is the equilibrium of torques – the relationship between the force of gravity pulling down on the mass with how far that mass is from the center. When the barbell is balanced at a point where those torques are equalized, it stays at rest and doesn’t fall off the stand.

A brass barbell, with a large weight on one end of the rod and a small weight on the other, rests at equilibrium on an aluminum stand 

 You can try this out in the classroom, at home, or anywhere else! You just need a long rod with different weights at either end - try a broom, a screwdriver, or a pencil with a big stick-on eraser.