gravity, vacuum and sum of forces

[m441]

Цитата:

it should be slown down

sure, but like center of gravity can't move (liquid+column+vacuum), if I move down vacuum object and move up liquid, I "see" the center of gravity move. With a lot cycles it's possible to image slow down far from Earth so I think I forget something

[Munin]

m441 в сообщении #771447 писал(а):

but like center of gravity can't move (liquid+column+vacuum)

This is wrong, because you can apply external forces to this system.

[m441]

no I don't apply external force, sorry I forget the spring: it is attached on column and vacuum object, system is : Spring+column+liquid+vacuum object

[Munin]

m441 в сообщении #771466 писал(а):

no I don't apply external force

So now you consider the water column in space without Earth? Then, of course, the column would shift, and the water would not (as a result).

[m441]

No, all the system is on Earth, on a table for example, the weight of the system is compensate by a force from the table. All is stable until the spring is attrack very quickly the vacuum object and the column (with liquid inside), vacuum object move down and liquid move up like column (because the spring is attached to the column). Why the system {spring+column+liquid+vacuum} don't move up from the table ?

[Munin]

m441 в сообщении #771486 писал(а):

No, all the system is on Earth, on a table for example

Then you have an external force from the table. And you cannot assume it constant.

If the motion is quick, then the system can jup a little on the table. (It is a good school problem to find the exact conditions of jump.) If it is not, the system just stays.

[m441]

Ok, if the system jump a little, if you take step by step the system, step 1-2: system jump a little, step 3-4: system jump a little, etc. the system don't change center of gravity ? because for pass from step 2 to step 3, it's possible to "forget" vacuum object and restart at new position. Sure the system will not be on the table at step 2 and must move down due to gravity, but if spring (or something else more efficient) is strong enough and attrack very quickly I have the impression the system move up.

[m441]

In my study (theoretical study), I suppose the viscosity at 0 (perfect fluid), like that it's easier to study.

[Munin]

Sorry I took it wrong. The system would jump if the water were going douw in the tube, but it's going up. Then no jump, only more pressure from the table.

[m441]

it's not logical for me, because spring pull column to up and spring pull vacuum to down, why the pressure on the table is greater ? What force I forget ? If acceleration is big enough, why F2 can't be greater than P ?



-- 07.10.2013, 01:49 --

I understood I think, P is the weigth, when vacuum object is put in water this increase pressure at bottom dp, but if I want the vacuum object stable the -dp is retrieve on column, if spring pull vacuum this give F1 to vacuum object, F1 give F1' pressure from dynamic pressure, but it give -F1 to the column, like that all weight is always P in static or in dynamic (I don't think it's possible to change weigth without external force ??), but the center of gravity has moved a little, what's force I forget ?

[m441]

If weight of the all system increase when vacuum object move down in the column, this would say the all system can give more energy when it move down than it lost potential energy like it's not possible, the weight must at P I think all the time, but in this case center of gravity move

[Munin]

m441 в сообщении #771755 писал(а):

it's not logical for me, because spring pull column to up and spring pull vacuum to down, why the pressure on the table is greater ?

If the system was in the space, then the spring would not pull the column to up. Instead, the water would stay on the spot, and thus the container would move down.

When this happens on Earth on the table, the container cannot move down because the table prevents that. So, the system would put an additional pressure on the table instead.

[m441]

In Atmosphere (with gravity). If system is at constant vertical velocity V. When the spring start to pull, if a little force is added to the bottom, this could say I recover more energy than I lost potential energy, no ?

[Munin]

m441 в сообщении #771838 писал(а):

In Atmosphere (with gravity). If system is at constant vertical velocity V.

What maintains that velocity?

[m441]

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