Poof -> Foop

[loic2003]

Here's a good one, if it's not a repost:

You have a cylinder lying on the ground filled with pressurised gas. You create a hole in one end (creation on the hole is assumed to be created instantly and would not affect the experiment). The gas escapes and the can moves in the opposite direction.

Question: same cylinder, same hole being opened, but this time there is a vacuum inside the cylinder. Does it move, and if so, in which direction?

Assume no friction between can and ground for simplicity.

 

[maximus maximus]

does not move.

 

[leftyman]

dont know, but it made me think of this.

 

[djheater]

I don't know enough about physics to give a smart answer, but it would seem to me that the internal pressure would try and equalize with the external, since external will be greater than internal there would not be any energy to move the cylinder.

 

[newmachineoverlord]

It moves in the direction of the hole because the air pressure on the opposite side will be greater than the pressure over the hole (which will be zero at the moment the hole is created and rapidly equalize).

 

[DivideBYZero]

Is it on a conveyor belt?

 

[BrownTown]

Originally posted by: newmachineoverlord
It moves in the direction of the hole because the air pressure on the opposite side will be greater than the pressure over the hole (which will be zero at the moment the hole is created and rapidly equalize).

newmachineoverlord FTW!!

Think about why a pressurized can moves, the pressure on the lside without the hole is greater than the side with a hole, so it moves the opposite direction of the hole. Now If you add a holw and a vacuum the higher pressures are on the outside, and the pressure on the side with the hole is more (but this time the direction is the opposite since its commign from the outside), so the can moves towards the side with the hole.

 

[NuroMancer]

Originally posted by: newmachineoverlord
It moves in the direction of the hole because the air pressure on the opposite side will be greater than the pressure over the hole (which will be zero at the moment the hole is created and rapidly equalize).

 

[beemercer]

Originally posted by: DivideBYZero
Is it on a conveyor belt?

🙂.

 

[Gibsons]

This reminds me of the sprinkler in reverse question.

 

[Paperdoc]

Yes, it moves towards the hole. IF you had a truly friction-free system it would be easy to see it start moving. In fact, in a truly friction-free system it would accelerate while the air was being sucked in, then just keep on moving until it hit something. But in many real-world systems the sucking-in-air-part will last only a short time and create a relatively small force, so the real friction might be sufficient to prevent perceptible movement and people will say "no movement".

A cylinder full of compressed gas at high pressure would contain a lot of gas so that the forces as it escapes are large and last a long time, making it easier to observe the effect. A vacuum, on the other hand, can only be one atmosphere (about 15 psi) less thant the air outside - there is no air pressure below zero! To make the two cases exactly comparable, the pressurized cyclinder would have to start out at a pressure of only 15 psi above ambient. Then its effects would be weak and short-lived, just like the vacuum scenario.