gravity, vacuum and sum of forces

[m441]

With starting constant V=1000 m/s, if a force Fs is added from {spring+liquid+vacuum+column} when spring pull, this force give energy for a time dt, this time at V=1000 m/s give a great work dt*V*Fs, it's big if we compare with V=10 m/s.

[Munin]

m441 в сообщении #771849 писал(а):

an external system, it can recover energy and control speed, it can be theoretical

I lost the last idea of your half-phrases. Sorry.

[m441]

you're asked what's control speed, so the system can be an electromagnetic system, this system recover energy and control the speed

[Munin]

Still cannot understand. Please describe your problem from beginning to end.

[m441]

Theoretical study. No friction. No temperature. Fluid is perfect: no viscosity. Under gravity (near a planet) but without atmosphere for cancel friction of air.

I take a column with liquid inside, with vacuum object at top position, a perfect spring pull vacuum object to bottom of the column when I want. I would like study the center of gravity and the weight.

I move up at 10000 m of altitude the {column+liquid+spring+vacuum}, I accelerate (down direction) the system (with vacuum at top) for have vertical down velocity at V=100 m/s. The system move down but a mechanism limit speed at 100 m/s (for don't have acceleration: easier to study). Now, the spring pull vacuum, liquid move up, vacuum move down, and you said P is greater due to the fact that liquid move up and the center of gravity change I think. So the additionnal force Fs, while a time dt give a work like: Fs*dt*V. But if speed is at 10 m/s the work giving is not the same (for me). It's for that I think the weight is always the same. But if the weight is the same (no additionnal force), when I move down vacuum and move up water, this change the center of gravity I think.

I hope it's cleaner, sorry if my english is not perfect.

[Munin]

m441 в сообщении #772246 писал(а):

The system move down but a mechanism limit speed at 100 m/s (for don't have acceleration: easier to calculate).

Then, the most important part is that mechanism. Every unbalanced external force would come from that mechanism. (System staying on the table would be simpler to talk about, and the same by physics.)

While the spring contracts, it moves the column down with respect to the water. Since the column cannot move down faster than its constant speed, then the column would be supported by the mechanism, and would receive an additional external force from it. And that force would move the center of mass.

m441 в сообщении #772246 писал(а):

But if speed is at 10 m/s the work giving is not the same (for me).

That's no problem. The work is always different for different speeds and reference frames. The only thing that keeps the same is balance. If there is an energy balance for one reference frame, then it would be for another. If there is no balance, then no. If some energy have become heat, then the heat would be the same in another reference frame. (Heat may change if you change the actual speed of motion, not the speed of reference frame.)

m441 в сообщении #772246 писал(а):

It's for that I think the weight is always the same. But if the weight is the same (no additionnal force), when I move down vacuum and move up water, this change the center of gravity I think.

You cannot move the center of mass without changing the weight (or without applying any other external force).

There is a theorem about the motion of the center of mass. The center of mass always move the same way as if there would be a point mass at its position, and all external forces would be applied directly to that point mass.

I recommend you to refer to the textbooks on Theoretical Mecanics (Analytical Mechanics, Classical Mechanics). For one example, there is a (popular in Russia)
Landau, L.D.; Lifshitz, E.M. Mechanics (Course of Theoretical Physics, Vol. 1).
Any other good textbook will do. (I am sorry for naming none, I am not familiar with textbooks in English or French.)

[m441]

Цитата:

and would receive an additional external force from it

the system can recover energy from this force Fs while time dt, with constant velocity V how energy recover is not Fs*dt*V ?

[Munin]

Please use TeX for formulas.

Of course, the energy received is $FV\,dt.$ Why not?

[m441]

the energy is very high if V is high, where the system lost energy ?

[Munin]

The idea of the question is wrong. System did not lose any energy. It has its kinetic energy and other kinds, whatever they are.

You just have to write down thoroughly:
- what energies there were at the beginning
- what energies were given to the system, or taken from it
- what energies are left at the end
and you will always get a perfect equality
$\displaystyle\sum E_b+\sum\Delta E=\sum E_e.$

[Nomads]

Почему нет дублирования на русском языке?

[Munin]

Потому что рабочие языки форума - русский, английский. Считается, что по-английски все понимают, и могут, если надо, по-английски говорить.

В данном случае, m441 из Франции, и может общаться по-английски лучше, чем по-русски (там был гугль-транслейт, ужос).

m441, we are talking about the language of this talk.