### moment of inertia factor

(characterization of mass distribution within a planet)

A body's **moment of inertia factor** is a measure that
characterizes the mass distribution within the body,
of use in working out the dynamics of bodies' rotation,
useful for objects such as stars, planets, and moons.
It is essentially that factor of the object's
**polar moment of inertia** that is
independent of the object's mass and radius:

C/MR²

- C - polar moment of inertia.
- M - mass.
- R - radius.

Such a value would be of interest regarding the rotation history of
objects, such as the timescale necessary for tidal forces
to produce tidal locking.
Example values:

sphere of uniform density | .4 |

object with higher density toward the surface | > .4 |

object with higher density toward the center | < .4 |

Sun | .070 |

Mercury | .346 |

Earth | .3307 |

Moon | .3929 |

Mars | .3662 |

Jupiter | .254 |

Saturn | .210 |

Uranus | .23 |

Neptune | .23 |

A small number indicates a lot of mass toward the center,
i.e., a dense "core", and both a higher total mass
and lower rigidity would contribute to this.

The **moment of inertia** of an object is a tensor indicating
its resistance to rotation, i.e., what force it takes to
change its rotation (much like the way mass determines what
linear acceleration results from a given force).
It is more than a scalar so as to include sufficient
information to characterize such resistance regarding
rotation around any axis through its center of mass.
A **polar moment of inertia** of an object is a scalar characterizing
the object's moment of inertia's implied resistance around a specific
axis. For astronomical bodies, the body's axis of rotation is what
is of generally of interest.

(*physics,measure*)
http://en.wikipedia.org/wiki/Moment_of_inertia_factor

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