Poof and Foop ( Dr. H Under Pressure)?

Question

This is a stumper. If a can of compressed air is punctured and the escaping air blows to the right, the can will move to the left in a rocket-like fashion. Now consider a vacuum can that is punctured. The air blows in the left as it enters the can. After the vacuum is filled the can will:

a) be moving to the left
b) be moving to the right
c) not be moving

Choose only one answer. The person who can explain their reasoning and selects the correct answer will be selected for Best Answer and all the immortal glory which accompanies it! Good luck.

Answer 1

I have to go with C as well.

I work with Vacuum systems day and day out. I deal with leaks all the time but have never seen a small chamber turn into a rocket or even move slightly.

I also work with gas cylinders and am well aware of their potential to turn into rockets.

hmmm....
My guess is that atmospheric pressure exerts an equal force over the can. P=F/A Even if a pin size hole is created the same force per unit are is acting on that hole as the whole can. So there will be no net force and therefore no movement.

In the case of a pressurized gas cylinder: Atmospheric pressure is exerting the same force per unit area all over the outside of the cylinder. The inside of the cylinder is experiencing lets say 2000 psi. But when a pin size hole is created the pressure at the hole is much greater then atmospheric pressure trying to hold the cylinder in place. So there is a Net force and thus movement.

I'm sticking with C because that's what I observe. I do concede that if we were to apply conservation of linear momentum the can should move to the left. But I don't think you can apply it here because it is not an isolated system due to the presence of external pressure.

Answer 2

I go for C also. It's about momentum. In expelling the pressurized air the pressure imparts momentum to the jet which results in a highly directional flow that transfers momentum to the atmosphere. Does this work backward? Does a vacuum at an orifice produce a jet flowing the other way? Nyet! The orifice collects air which is replaced by an essentially radial inward flow. No momentum change to the atmosphere so none to the can.

Answer 3

After due consideration of the principles of thrust, conservation of momentum, and impulse, I choose answer B, moving to the right.

[In response to the fellow who commented that he works with compressed gas and vacuum bottles, but has not observed them moving when their valve is opened. Consider what happens when you drop an aluminum can of soda pop. When it punctures, it goes through crazy contortions as the carbon dioxide gas escapes. Industrial compressed gas and vacuum bottles do not behave like that unless there is a lot of gas movement, such as the bottle valve breaking off.]

Answer 4

I think the can will move to the left as when the air rushes into the can it will be creating a push to the right with this push being equal to the suction power to the left however once the pressure in the can equalizes the can will move to the left as the suction will only stop once the can is full leaving the push to the left as the only remaining force.

Answer 5

Due to the equalisation of pressure, there's no force acting in either direction...the answer will be 'c)'.

(Prior to that, the can will move towards the air inlet due to the 'suction' created by the vacuum).

Answer 6

not be moving i think.

Does the pressure on the outside equalise the pressure on the inside.

Here can you help me with some of my Physics problems, you seem like a smart man

Answer 7

i'm just guessing that it don't move at all.
hhope this is right, and sorry i don't know why nor how.
i hope i got it right