Rotational kinetic energy decreases, internal increases?

[Gian_ni]

If rotational kinetic energy of a closed system decreases, another form of energy must increase for the conservation of energy of a closed system.
We assume this system (a person in rotation) has these forms of energy:
ΔE=ΔK (only rotational around an axis) + ΔUi (internal) + ΔEt (thermal) with ΔE=0, ΔK<0
The muscles make always an effort so also ΔUi<0. Then ΔEt>0 Is it right?

But if ΔK>0 the muscles make an effort ΔUi<0. ΔUi = -ΔK ( ΔEt=0 )

There is a contradiction in the first case so that ΔUi should be > 0 ?

Thank you

 

[mfb]

Muscles? Do you have some specific system in mind?

The muscles make always an effort so also ΔUi<0. Then ΔEt>0 Is it right?

Probably. It is not clear what exactly you consider.

But if ΔK>0 the muscles make an effort ΔUi<0. ΔUi = -ΔK ( ΔEt=0 )

In general there will still be friction, and the muscles have to provide energy both for friction and for increasing the kinetic energy.

 

[Dale]

Biological systems are messy. When in doubt, choose a simpler system. Here, I would recommend a pair of masses attached to a spring of rest length L. You can see changes in KE and internal energy. Later, if you want to add changes in thermal energy you can make it a damped spring by adding a dash pot.

 

[Gian_ni]

Muscles? Do you have some specific system in mind?

Yes, muscles of a person that change his moment of inertia..
So, if ΔK is <0 why isn't ΔUi>0 ? Certainly we know that if ΔK>0 ΔUi<0 ( the person has dissipated energy )

 

[jbriggs444]

Yes, muscles of a person that change his moment of inertia..
So, if ΔK is <0 why isn't ΔUi>0 ?

It would be, except that muscles do not harvest energy when they are expanded while under tension. If you run down a mountain side, it does not put glucose back into your bloodstream. The lost energy will almost certainly wind up as heat. Wasted energy usually does.