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A Dynabee, also known as Gyrotwister, is a gyroscopic toy sold commercially as a wrist exerciser device. Once the gyroscope inside it is going fast enough, for example after starting it with a short rip string or by a snap of the thumb, a person holding it in her hand can accelerate the gyroscope to incredibly high revs by following a circular wrist motion with the device. Modern devices come with electronic rev counters and light emitting diodes powered by a little generator embedded in the gyroscope.

The physics of this device looks confusing at first sight, but is surprisingly simple. It can easily be understood with a little knowledge of gyroscopes. Their striking property is that the axis of a spinning gyroscope will resist a force applied to it and will start rotating perpendicular to the force applied. This is why spinning gyro tops will not simply fall over, but start precessing around. (The precessing motion is perpendicular to the torque trying to make the gyro fall over.)

The axis of the gyroscope in the Dynabee is fixed to the spinning mass and it rests in a little groove inside the wrist exerciser device, which almost completely covers the gyroscope inside it, except for a small round opening on top of it, which is where you can manually start the gyroscope. Once the gyro is spinning, tipping the device will cause the gyroscope to start precessing, with its axis slipping around in the groove in a circular fashion. The groove inside the device, is a little wider than the axis, and the gyroscope's evasive action towards the externally applied force will cause one end of the axis to push against the upper rim of the groove, while the other end of the axis pushes against the lower rim of the groove. While the axis is slipping around inside the groove, the friction between the axis and the groove rims will accelerate or brake the spinning gyroscope, with a maximum effect when the axis starts "rolling" inside the groove. (Compare with graphic below.) Since this friction force is essential for the device's operation, the groove must not be lubricated. The acceleration of the gyroscope is best when the precession of the gyroscope is supported and amplified by wrist motion.

It takes a while until one finds the "rolling" point, but the gyro will also be accelerated to a smaller extent by the slipping friction.

The device is covered by US patents 3,726,146 (1973) and 5,353,655 (1994) by L.A. Mishler and US patent 5,800,311 (1998) by P.S.Chuang.

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