# Thread: thermodynamics in a box

1. Hi to all physicum passionates,

I was wondering a simple fact,
Imagine you have a box in the space filled with gas and you have a cube inside the box that you can put in movement with magnet outside the box.
Imagine the cube is put in movement for a certain duration T to a constant velocity V.
With the front face In each collision the gas momentum velocity will ingress in the direction of the cube velocity V.
But with the back face the inverse effect will occure.
So why does it require energy to make it moving forward ?

2.

3. Originally Posted by manoir
Hi to all physicum passionates,

I was wondering a simple fact,
Imagine you have a box in the space filled with gas and you have a cube inside the box that you can put in movement with magnet outside the box.
Imagine the cube is put in movement for a certain duration T to a constant velocity V.
With the front face In each collision the gas momentum velocity will ingress in the direction of the cube velocity V.
That is not how things move through air. Pressure builds up in front of the block and the air will flow to the side. This results in a bow wave.
But with the back face the inverse effect will occure.
At the back of the block there will be a low pressure area.
So why does it require energy to make it moving forward ?

If you stop applying a force to make it move, the high pressure in the direction of travel and the low press in the back will both stop the movement of the block. Not to mention the friction from the ground.

4. yes thank you for your help it s reversible only with "an infinity slow deplacement"

5. My question was more if you have helium gas in a bottle, when you have a collision of a helium molecule with another one, it s reversible so the displacement of a helium molecule in helium gas is reversible. But the displacement of a macroscopique object in the gas is not. From which dimension the displacement becomes to be non reversible ?

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement