gravity, vacuum and sum of forces

[m441]

When a bubble of air move up in water (on Earth), Earth move in the other direction ?

[Munin]

Yes.

[вздымщик Цыпа]

Munin в сообщении #771092 писал(а):

Yes.

Разве? Если пузырь воздуха движется вверх, то равный ему объем воды движется вниз. Вес воды больше веса пузыря, значит Земля должна двигаться навстречу воде и в направлении движения пузыря. Нет?

[Munin]

You're right. Of course, Earth moves in the same direction!

[m441]

and the center of gravity move or not ?

[Munin]

What is the center of gravity in this context?
The center of mass (of Earth + water + bubble) does not move.

[m441]

Ok, but there is something I don't understand.

New case: On Earth with gravity. I take a column with liquid inside. At top, there is a vacuum object. A system fixed to the column move down the vacuum object. The vaccum object move down, the column move up, the cycle can be repeat because I don't move up vacuum object, so I think the column (with liquid inside) can move up. I never move out vacuum object, I repeat cycle with bottom water.

I know it's not possible so what's another force I forget ?

[m441]

i add image

[Munin]

m441 в сообщении #771301 писал(а):

...so what's another force I forget ?

Buoyant force? :-)

-- 06.10.2013 12:10:05 --

(The work against the buoyant force is exactly the same as to move the whole column of water up to the difference of heights.)

[m441]

I don't know. Take a column of water of 10 kg, the vacuum object is 1 liter, the weight of the system is always 10 kg. This object has an acceleration of 9.81 m/s². If I move vacuum object with an acceleration greater than 10 m/s² why column would move down ?

[Munin]

m441 в сообщении #771331 писал(а):

If I move down vacuum object, liquid move up

Yes again.

[m441]

sorry, I changed my last message:

Take a column of water of 10 kg, the vacuum object is 1 liter, the weight of the system is always 10 kg. This object has an acceleration of 9.81 m/s². If I move vacuum object with an acceleration greater than 10 m/s² (in fact I think greater than 100m/s² because vacuum object represent 10% of the volume) why column would move down ?

[Munin]

Accelerations? I thought you are considering the system on the surface of Earth.

[m441]

yes, the study is on Earth. But I use a spring for example for move down the vacuum object. The spring is attached at one part on column and another part to vacuum object.

[Munin]

It is easier to consider just works and energy, and not particular motions. If something is accelerated, it should be slown down. Better think of applying the force just enough to drown the object, without any acceleration, and of motion slow enough to neglect accelerations ang kinetic energy.