The following quote comes from: https://physics.stackexchange.com/a/93531/155365
Consider an ice skater that has just gone into a spin with arms stretched out. If it helps, the skater is wearing lead bracelets on each wrist. As the skater spins, the angular momentum and angular kinetic energy are constant; friction with the air and with the ice can be eliminated. The skater must exert an inward centripetal force on the bracelets to keep them rotating in the same circle, but this force does no work, since the bracelets are not changing their radius of rotation.
Then the skater pulls in his arms and the bracelets, closer to his axis of rotation.
Several interrelated things happen:
- since there is no external torque, the angular momentum of the system stays the same;
- Since the various masses of the skater are all moving towards the axis of rotation, the total moment of inertia of the system decreases;
- Combining 1 and 2, the angular velocity of the system increases; if the moment is halved, the angular velocity doubles
- Combining 2 and 3, with some quantitative treatment, the angular kinetic energy increases; if the moment is halved and the angular velocity is doubled, the kinetic energy doubles;
- The skater does work; his arms are now exerting an inward force through a distance as his arms draw the bracelets inward; the work done can be rigorously shown to be identical to the increase in kinetic energy...
However, it only covers a case when work is done to pull arms inwards. It is understandable that the work would be done in such case.
But what is happening in the opposite case when the skater has their arms inwards and lets them go outwards? S/he doesn't do any work but kinetic energy (and total energy) is lost. Why is it lost?
I = 2kg * (1m)^2 w = 3.14/s L = 2kg * m^2 * 3.14/s Ek = 9.869 After shifting 2kg by 0.1m outwards: I = 2kg * (1.1m)^2 = 2.42 kg * m^2 L = 2 * 3.14 = 2.42 * w w = 2.5963 Ek = 8.1566
So, where did 1.7124 J go?
The quoted answer describes the easy case and omits the hard one.