Is it possible to calculate the moment of inertia of a disk that is rotating around its centre and simultaneously circling around another axis that is paralled to the one in the cente.

For example... Earth is rotating around the sun (first axis) and around the centre (second axis).

How would I calculate moment of inertia?

Is it even possible?

  • 2
    $\begingroup$ Look up the parallel axis theorem for moment of inertia. $\endgroup$
    – Bill Watts
    Commented Jan 11, 2021 at 3:18
  • $\begingroup$ Nooo that was not my question $\endgroup$ Commented Jan 11, 2021 at 5:54
  • $\begingroup$ Parallel axis theorem. $\endgroup$
    – Kashmiri
    Commented Jan 11, 2021 at 8:06
  • $\begingroup$ Well, the question you asked has an answer that is provided by a very simple application of the parallel axis theorem. $\endgroup$
    – Bill Watts
    Commented Jan 11, 2021 at 8:14
  • $\begingroup$ Possible duplicate by OP: physics.stackexchange.com/q/606848/2451 $\endgroup$
    – Qmechanic
    Commented Jan 11, 2021 at 16:47

1 Answer 1


You need to calculate the moment of inertia of the two systems separately. For rotation about own axis, you will have one inertia. For rotation about Sun you will have one moment of inertia. Moment of inertia is always defined with respect to axis of rotation.

But yes, the object will have a energy or angular momentum which is a summation of energy and angular momentum of individual systems (own axis and object+external point) and that total value would be a constant value by laws of conservation of angular momentum and laws of conservation of energy.

Moment of inertia as a whole like energy or momentum is not defined.


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