How this calculator works
Rotational kinetic energy adds the kinetic energy of all mass elements moving around the selected axis. Their radial distribution is represented by moment of inertia. Angular speed is squared, so energy is independent of rotation direction. If the object also translates, add its centre-of-mass translational kinetic energy separately.
Formula
E_rot = I omega²/2; omega = sqrt(2E_rot/I); I = 2E_rot/omega².
Worked example
A rotor with I = 4 kg m² at 3 rad/s stores 18 J. A 2 kg m² rotor storing 100 J turns at 10 rad/s.
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Frequently asked questions
Can I enter rpm?
Yes. Select revolutions per minute for the angular-speed input; calculations convert it to radians per second.
Is moment of inertia just mass?
No. It includes distance squared from the axis and has units kg m².
Why is zero speed unsuitable for finding inertia?
Every inertia has zero rotational energy at rest, so those measurements do not determine inertia.