Rotating Saddle

A playground ball finds stability in a saddle when the saddle is rotating at the proper speed. Mechanical analog of a “Paul Trap“ particle confinement—a ball is trapped in a time-varying quadrupole gravitational potential. A large saddle shape (attached to a plywood disk) is mounted on a multi-purpose turntable. The saddle shape is essentially a quadrupole gravitational potential. Rotation of this potential subjects the ball to an alternating repulsive and attractive potential, much like the time-varying electric quadrupole potential of a Paul Trap used in trapping single ions or electrons. The plastic ball used here is about 25 cm in diameter and was purchased at a toy store. The saddle consists of a rubber sheet and fiberglass, and was hand-made with help from Justin Georgi. The turntable is driven at about 110 rpm with a DC motor. We have observed this ball at this speed remaining stable for over 2 hours. Slow motion footage recorded at 240 fps with a Casio EX-FH25. Thanks to Rob for letting us use the camera. For more information on our demo and references for further study see