Rotational kinetic energy decreases, internal increases?

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Discussion Overview

The discussion revolves around the relationship between rotational kinetic energy and internal energy changes in a closed system, particularly in the context of a person rotating. Participants explore the implications of energy conservation when rotational kinetic energy decreases and the role of muscles in this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that if the rotational kinetic energy (ΔK) of a closed system decreases, then another form of energy, such as internal energy (ΔUi) or thermal energy (ΔEt), must increase to conserve energy.
  • It is proposed that if ΔK is less than zero, then internal energy (ΔUi) must also be less than zero, leading to an increase in thermal energy (ΔEt) to maintain energy conservation.
  • Another participant questions the assumptions about muscle energy expenditure and suggests that if ΔK is greater than zero, then ΔUi must be less than zero, indicating energy dissipation.
  • A different perspective is introduced, recommending a simpler system (a pair of masses attached to a spring) to analyze changes in kinetic and internal energy, which may clarify the energy dynamics involved.
  • One participant emphasizes that muscles do not harvest energy when they are expanded under tension, suggesting that energy lost in this context likely converts to heat rather than being stored as internal energy.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between rotational kinetic energy and internal energy changes, particularly regarding the role of muscles and energy dissipation. The discussion remains unresolved, with no consensus on the implications of energy conservation in this context.

Contextual Notes

Participants note the complexity of biological systems and the assumptions made about energy transformations, particularly regarding muscle function and energy dissipation. The discussion highlights the need for clearer definitions and a more straightforward system for analysis.

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
 
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Muscles? Do you have some specific system in mind?
Gian_ni said:
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.
Gian_ni said:
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.
 
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.
 
Last edited:
mfb said:
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 )
 
Gian_ni said:
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.
 

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