Physics

I learned this week that many high speed CD-ROM drives used balancing balls on the spindle to stop discs from vibrating at 10Krpm.

Between the platter that supports the CD and the motor there is a puck with a toroidal void containing a few ball bearings. When an out of balance CD is spun up the spindle and disc together rotate around their common center of mass, some point between the spindle and the edge of the disk. This means that the void containing the balls no longer rotates around it's center, it spins like a hula-hoop around the spindle/DC center of mass. With the "lighter" side of the system being farther from the center of rotation the balls roll 'down hill' towards the side of the void that is experiencing more centrifugal force. Eventually enough balls will collect on the light side to perfectly cancel out the heavy side. If there are too many balls they will distribute themselves inside the void until they cancel out each other's weight!

The link leads to a scaled up demo of this using an empty water bottle and steel BBs.