Also known as **Tennis Racket Theorem**.

The **tennis racket theorem** or **intermediate axis theorem** is a result in classical mechanics describing the movement of a rigid body with three distinct principal moments of inertia. It is also dubbed the **Dzhanibekov effect**, after Russian cosmonaut Vladimir Dzhanibekov who discovered the theorem's consequences while in space in 1985. An article explaining the effect was published in 1991.

The theorem describes the following effect: rotation of an object around its first and third principal axes is stable, while rotation around its second principal axis (or intermediate axis) is not.

This can be demonstrated with the following experiment: hold a tennis racket at its handle, with face horizontal, and try to throw it in the air so that it will perform a full rotation around the horizontal axis perpendicular to the handle, and try to catch the handle. In almost all cases, during that rotation the face will also have completed a half rotation, so that the other face is now up. By contrast, it is easy to throw the racket so that it will rotate around the handle axis (the third principal axis) without accompanying half-rotation around another axis; it is also possible to make it rotate around the vertical axis perpendicular to the handle (the first principal axis) without any accompanying half-rotation.

The experiment can be performed with any object that has three different moments of inertia, for instance with a book, remote control or smartphone. The effect occurs whenever the axis of rotation differs slightly from the object's second principal axis; air resistance or gravity are not necessary.

See Also

**axis**
**axis of gravity**
**axis of rotation**
**concept in inertia**
**Figure 10.03 - Zero Planes of Depolar Inertia**
**Figure 12.07 - Plane of Inertia showing Focalizing Action**
**Figure 12.08 - Plane of Inertia shown as an Optical or Focalizing Function**
**Figure 3.24 - Non-synchronized Voiding at Plane of Inertia is Regenerative**
**Figure 5.11 - Dynamics and Development of Circulating Contractive Ring on One Axis**
**Figure 6.2 - Opposing Repellant Dispersive Radiations Neutralizing at Interface Plane of Inertia**
**Figure 7.2 - Step 2 - Vortex Formation about a Single Axis**
**Inertia**
**inertial plane**
**Motion-in-inertia**
**non-motion-in-inertia**
**Plane of Inertia**
**triple inertia planes**
**wave axis**
**wobbling axis**
**3.7 - Non-synchronized Voiding at Plane of Inertia**