What are the forces acting on the container?

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Let’s say I bring a closed container full of air into deep space where atmospheric pressure is is near 0. I Open the lid and then the air moves out of the container and into the vacuum of space. What are the forces on the container AFTER the lid has been removed? Does the container move, ignoring any force used to remove the lid?
Let’s say I bring a closed container full of air into deep space where atmospheric pressure is is near 0. I Open the lid and then the air moves out of the container and into the vacuum of space. What are the forces on the container AFTER the lid has been removed? Does the container move, ignoring any force used to remove the lid?
 

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  • #2
scottdave
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Did the air molecules move? Think about conservation of momentum.
 
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Did the air molecules move? Think about conservation of momentum.

Yes the air molecules move. “Conservation of momentum is a fundamental law of physics which states that the momentum of a system is constant if there are no external forces acting on the system. It is embodied in Newton's first law (the law of inertia).”

The external force is provided by pressure gradient force. This force causes fluids to move from high pressure to low pressure.
https://en.m.wikipedia.org/wiki/Pressure-gradient_force
Therefore conservation of momentum satisfied.

Additionally Joules-Thomson states that fluids expand freely in a vacuum without doing any work.
http://physics.bu.edu/~duffy/semester1/c27_process_expansion_sim.html
Taking all this into consideration, what are the forces acting on the container after the lid is opened?
 
  • #4
Tom.G
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Just for the sake of discussion, let's make the container a cube.

1) What are the forces on the 6 surfaces of the container? Draw a sketch of the forces.
2) When one surface, the 'lid', is removed there are 5 remaining surfaces.
3) At the instant the lid is removed, while there is still some air in the container, draw another sketch showing the forces on the remaining surfaces.

Are there any forces on the container that are not balanced by another force opposing it?

Cheers,
Tom
 
  • #5
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Just for the sake of discussion, let's make the container a cube.

1) What are the forces on the 6 surfaces of the container? Draw a sketch of the forces.
2) When one surface, the 'lid', is removed there are 5 remaining surfaces.
3) At the instant the lid is removed, while there is still some air in the container, draw another sketch showing the forces on the remaining surfaces.

Are there any forces on the container that are not balanced by another force opposing it?

Cheers,
Tom

force applied on all six side equal to pressure*surface area.

When lid is opened, pressure decreases and the force on the remaining walls also decreases.

Like when you need to fart. You sphincter opens and pressure rushes out giving instant relief to the walls of the bladder.
 
  • #6
phinds
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force applied on all six side equal to pressure*surface area.

When lid is opened, pressure decreases and the force on the remaining walls also decreases.

Like when you need to fart. You sphincter opens and pressure rushes out giving instant relief to the walls of the bladder.
Yes, but you haven't answered his question. He is trying to help but you are not thinking through what he asked you to think through (and exactly in the way he suggested)
 
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Yes, but you haven't answered his question. He is trying to help but you are not thinking through what he asked you to think through (and exactly in the way he suggested)
Yes I did answer the question. Obviously there is air in the container when lid is open. I further explained that as soon as the lid is opened, volume is increased and pressure is decreased. thus the forces applied to the wall is decreased and the overall momentum of the air is moving out of the container.

PV=nRT

When the lid is opened, volume is increased to essentially infinity. The pressure then decreases to essentially 0

If pressure is 0, forces on the walls is 0
 
  • #8
phinds
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Yes I did answer the question. Obviously there is air in the container when lid is open. I further explained that as soon as the lid is opened, volume is increased and pressure is decreased. thus the forces applied to the wall is decreased and the overall momentum of the air is moving out of the container.

PV=nRT

When the lid is opened, volume is increased to essentially infinity. The pressure then decreases to essentially 0

If pressure is 0, forces on the walls is 0
I don't feel that you have. Certainly, none of that gives an explicit answer to you own question. Does the box move?
 
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I don't feel that you have. Certainly, none of that gives an explicit answer to you own question. Does the box move?
I’m the one who asked the question. I have you an explanation of my understanding. Is right or wrong and why?
 
  • #10
phinds
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I’m the one who asked the question. I have you an explanation of my understanding. Is right or wrong and why?
We are trying to lead you to a correct understanding of the situation you have described. You continue to not answer your own question so I don't see how you can believe you have given an explanation. The question is, does the box move or not?

Perhaps it would help you to think of a slightly different scenario. Suppose the box has an airtight hole on one face the size of a dime and that hole slides open. What happens to the box and why?
 
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We are trying to lead you to a correct understanding of the situation you have described. You continue to not answer your own question so I don't see how you can believe you have given an explanation. The question is, does the box move or not?

Perhaps it would help you to think of a slightly different scenario. Suppose the box has an airtight hole on one face the size of a dime and that hole slides open. What happens to the box and why?
When the hole is opened the volume is increased to essentially infinity and pressure is decreased to essentially 0. Same scenario as the lid example.

Why don’t you tell what your understanding is?
 
  • #12
anorlunda
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When the hole is opened the volume is increased to essentially infinity and pressure is decreased to essentially 0.
What happens to the air that goes out the hole? How does that relate to motion of the box?
 
  • #13
256bits
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When the hole is opened the volume is increased to essentially infinity and pressure is decreased to essentially 0. Same scenario as the lid example
Well that is true.
First of all you have air in the box and some time later there is none.

People are asking you to think of what happens with the box between those two events, during which the air is escaping.
 
  • #14
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Well that is true.
First of all you have air in the box and some time later there is none.

People are asking you to think of what happens with the box between those two events, during which the air is escaping.

Well the gas moves out of the container because of pressure gradient force. Gas moves freely into a vacuum. So there are no forces acting on the container.
 
  • #15
phinds
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Well the gas moves out of the container because of pressure gradient force. Gas moves freely into a vacuum. So there are no forces acting on the container.
Yes, we understand that you have this misconception, what we are asking you to do is think it through. Forget about what it's like after all the air has left the container and think about what happens as the air leaves the container.
 
  • #16
phinds
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Why don’t you tell what your understanding is?
Because I'm trying to get you to think it through and get the answer for yourself. I consider it a waste of time to just spoon feed you an answer. This is not a Q&A forum, it's a forum where the focus is on teaching people to think, not on providing answers.
 
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Yes, we understand that you have this misconception, what we are asking you to do is think it through. Forget about what it's like after all the air has left the container and think about what happens as the air leaves the container.
The air moves in one direction out of the container. Just like when wind blows on the back of your head, the air in front of you moves away from you in one direction without applying a force on your face.
 
  • #18
256bits
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The air moves in one direction out of the container.
Why?
What is it pushing on?

PS -You ever play with a balloon, and let the air escape out of the small tube end.
 
  • #19
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Why?
What is it pushing on?

PS -You ever play with a balloon, and let the air escape out of the small tube end.
The force is provided by pressure gradient force not because the container physically throwing it out. https://en.m.wikipedia.org/wiki/Pressure-gradient_force

Joules-Thomson also explains that work is done external pressure is greater than zero. Meaning that the air coming out of the balloon pushes off the atmosphere pressure in order to create movement.

In space the external pressure is 0
 
  • #20
256bits
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The force is provided by pressure gradient force not because the container physically throwing it out. https://en.m.wikipedia.org/wiki/Pressure-gradient_force

Joules-Thomson also explains that work is done external pressure is greater than zero. Meaning that the air coming out of the balloon pushes off the atmosphere pressure in order to create movement.

In space the external pressure is 0
That is only going so far, in other words what would be the pressure gradient at the closed end of the box?
 
  • #21
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That is only going so far, in other words what would be the pressure gradient at the closed end of the box?
The pressure gradient at the closed end of the box is positive to space.
 
  • #22
phinds
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The air moves in one direction out of the container. Just like when wind blows on the back of your head, the air in front of you moves away from you in one direction without applying a force on your face.
Not even remotely the same kind of situation. The air moving out of the container is absolutely not "just like ... "
 
  • #23
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Not even remotely the same kind of situation. The air moving out of the container is absolutely not "just like ... "
What do you mean not “just like”. ? Explain further
 
  • #24
phinds
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What do you mean not “just like”. ? Explain further
What you are saying is EXACTLY the same as saying that if I am standing next to a guy who fires a gun, I feel no recoil (correct) but if I fire my gun, I ALSO feel no recoil (clearly incorrect).
 
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  • #25
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Why you are saying is EXACTLY the same as saying that if I am standing next to a guy who fires a gun, I feel no recoil (correct) but if I fire my gun, I ALSO feel no recoil (clearly incorrect).
Recoil from the gun is different. Pressure build between the bullet and the gun. The pressure pushes off both the bullet and then the gun. A rocket is like firing a blank where the gun powder is not encased in anything
 

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