This toy uses the principles of center of mass to keep balance when sitting on finger.
Sphere rotating around one of its diameters
Diagram of an educational toy that balances on a point: the center of mass (C) settles below its support (P)
An example of rotation. Each part of the worm drive—both the worm and the worm gear—is rotating on its own axis.
Plumb line method
Two bodies orbiting their barycenter (red cross)

The special case of circular orbits is an example of a rotation around a fixed axis: this axis is the line through the center of mass perpendicular to the plane of motion.

- Rotation around a fixed axis

Typically, a human's center of mass is detected with one of two methods: the reaction board method is a static analysis that involves the person lying down on that instrument, and use of their static equilibrium equation to find their center of mass; the segmentation method relies on a mathematical solution based on the physical principle that the summation of the torques of individual body sections, relative to a specified axis, must equal the torque of the whole system that constitutes the body, measured relative to the same axis.

- Center of mass
This toy uses the principles of center of mass to keep balance when sitting on finger.

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The position of a rigid body is determined by the position of its center of mass and by its attitude (at least six parameters in total).

Rigid body

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Solid body in which deformation is zero or so small it can be neglected.

Solid body in which deformation is zero or so small it can be neglected.

The position of a rigid body is determined by the position of its center of mass and by its attitude (at least six parameters in total).

The linear position can be represented by a vector with its tail at an arbitrary reference point in space (the origin of a chosen coordinate system) and its tip at an arbitrary point of interest on the rigid body, typically coinciding with its center of mass or centroid.

Two points of a rotating body will have the same instantaneous velocity only if they happen to lie on an axis parallel to the instantaneous axis of rotation.