**Ask a monkey a physics question thread**

[OngBonga]

mojo can you give me any examples of perfect equilibrium in nature? Every case that I can think of is near perfect equilibrium, such as the Solar System. It can't be perfect equilibrium because none of the bodies in the system are perfect spheres, meaning gravity interactions will fluctuate, and there are influences from beyond the system.

Is there even any such thing as a closed system? Other than the universe itself.

[MadMojoMonkey]

mojo can you give me any examples of perfect equilibrium in nature? Every case that I can think of is near perfect equilibrium, such as the Solar System. It can't be perfect equilibrium because none of the bodies in the system are perfect spheres, meaning gravity interactions will fluctuate, and there are influences from beyond the system.

You're getting lost in your head-space. Equilibrium is everywhere.

Anything which is not accelerating is experiencing static equilibrium. Dynamic equilibrium has multiple contexts, but is broadly analogous to a steady-state of a system. Meaning that the system undergoes changes, but more-or-less this is a cycle which represents a single energy-state.

When you sit on your chair, you accelerate your body in a complicated set of ways which culminate in you sitting still in your chair. This is equilibrium. If you want to nit-pick about your breathing and other biological process going on which are not altogether cyclical on this time scale, then we can certainly note that those parts of the system do not seem to be experiencing equilibrium. However, the system as a whole is in a steady state w.r.t it's position. I.e. the average change in velocity of all the bits of you is 0 to within experimental uncertainties.

Whenever we choose a model, we accept limitations, or boundaries of applicability. It is excellent form for you to question which parts of the system are behaving in which way.

Is there even any such thing as a closed system? Other than the universe itself.

It's all semantics. So long as you are accepting that stuff-n-things may play a role in putting boundaries on your control volume, you're free to run.

Consider a chemists experiment in a test tube. The closed system is pretty obviously the volume of the test tube, as far as the contents of the container and their chemical properties. If it is a well-insulated test tube, then that's one more property that can be considered to be isolated in the control volume.

IDK about the semantics, but control volume is the terminology used in vector mathematics. So long as you account for changes happening at the boundaries of the control volume, then it need not be a "closed system."

EDIT: This is the same for Thermodynamics. One of the fundamental equations of Thermodynamics is
Q_in - W_out = delta_E
The Heat in minus the Work out equals the change in energy of the control volume.

This tells you how to deal with the goings-on at the boundaries of the control volume.