What are the forces acting on the container?

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  • #1
Chuzzled
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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.
 
  • #3
Chuzzled
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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
Chuzzled
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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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  • #7
Chuzzled
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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?
 
  • #9
Chuzzled
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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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  • #11
Chuzzled
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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
Chuzzled
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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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  • #17
Chuzzled
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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
Chuzzled
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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
Chuzzled
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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
Chuzzled
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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
Chuzzled
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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
 
  • #26
phinds
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OK, @Chuzzled you have persistently refused to answer our questions or really think about all the many hints we have given you and your last post states pretty clearly that you don't think rockets can get off the ground. At this point I just think you are trolling us to see how long you can string us along.

I'm going to bow out of this thread. Perhaps others here will have more faith in your sincerity than I do.
 
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  • #27
Tom.G
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I'll give it one more try. Actually, @phinds had a good start on it in post #10.

As @Chuzzled said in post #5:
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.

Then in post #10 @phinds suggested:
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?
Condition: While there is still some air in the container after the hole is opened:
  1. The surface with the hole now has less surface area than the other surfaces.
  2. This, by your own (correct) statement in post #5, leads to less force on that surface than on the other surfaces.
  3. But the surface opposite the one with the hole still has its full area and the full force of the contained gas pushes against it.
  4. This leads to that opposite surface being pushed away from the surface with the hole.
  5. Consequently the box moves away from the hole. (More accurately away from the direction the gas is moving.)

Let's try it with some numbers. Since I'm used to English units, make the box a cube 10 inches on a side, making the surface area of each side 100 sq.in. With 15psi (15 pounds per sq.in.) there is 1500 pounds force on each side of the cube.

A hole the size of a U.S dime is about 0.7in. diameter, which makes the area 0.39 sq.in.

The area of the side with the hole is now 100 - 0.39 = 99.61 sq.in. and the force is 99.61 x 15 = 1494 pounds force. But the opposite wall still has 1500 pounds force on it.

This gives a net, unbalanced, 6 pounds force pushing the container away from the side with the escaping gas.

Hope this helps. It really is hard to wrap your head around until you dig into these details!

Cheers,
Tom
 
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  • #28
Chuzzled
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I'll give it one more try. Actually, @phinds had a good start on it in post #10.

As @Chuzzled said in post #5:


Then in post #10 @phinds suggested:

Condition: While there is still some air in the container after the hole is opened:
  1. The surface with the hole now has less surface area than the other surfaces.
  2. This, by your own (correct) statement in post #5, leads to less force on that surface than on the other surfaces.
  3. But the surface opposite the one with the hole still has its full area and the full force of the contained gas pushes against it.
  4. This leads to that opposite surface being pushed away from the surface with the hole.
  5. Consequently the box moves away from the hole. (More accurately away from the direction the gas is moving.)

Let's try it with some numbers. Since I'm used to English units, make the box a cube 10 inches on a side, making the surface area of each side 100 sq.in. With 15psi (15 pounds per sq.in.) there is 1500 pounds force on each side of the cube.

A hole the size of a U.S dime is about 0.7in. diameter, which makes the area 0.39 sq.in.

The area of the side with the hole is now 100 - 0.39 = 99.61 sq.in. and the force is 99.61 x 15 = 1494 pounds force. But the opposite wall still has 1500 pounds force on it.

This gives a net, unbalanced, 6 pounds force pushing the container away from the side with the escaping gas.

Hope this helps. It really is hard to wrap your head around until you dig into these details!

Cheers,
Tom

Can you prove that? As soon as you open the hole, pressure decreases and the force applied to all walls decreases as a result.

What you said cannot be observed in reality. Let’s say you have a pipe with only one end open. You put a small hole on the side of the pipe near the closed end. You use vacuum near the open end to create a pressure gradient cause air to move out of the pipe. Why doesn’t the pipe move forward or at least have some thrust?

Can you create a simple experiment proving what you have stated?
 
  • #29
gmax137
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As soon as you open the hole, pressure decreases and the force applied to all walls decreases as a result.

true, the effect is transient.

...Why doesn’t the pipe move forward or at least have some thrust?

because the engineers have included pipe restraints or hangers. if your pipe were free-floating in space you would see the motion.

Can you create a simple experiment proving what you have stated?
try the previously recommended toy balloon trick
 
  • #30
jbriggs444
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What you said cannot be observed in reality. Let’s say you have a pipe with only one end open. You put a small hole on the side of the pipe near the closed end. You use vacuum near the open end to create a pressure gradient cause air to move out of the pipe. Why doesn’t the pipe move forward or at least have some thrust?
You seem to be complicating the scenario unnecessarily. Close off both ends of the pipe. Create a small hole on the side of the pipe near one end. Assume the environment is vacuum. What happens next?

Does air blow out? If so, what does conservation of momentum say? If not, why not?

What forces exist on the inner walls of the pipe before the air has completely been exhausted out into the vacuum. Is there an imbalance?

On a different matter. The sum of the forces on the six sides of the box is not six times the force on anyone side. Forces are vectors. They add like vectors do.
 
  • #31
Chuzzled
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true, the effect is transient.



because the engineers have included pipe restraints or hangers. if your pipe were free-floating in space you would see the motion.


try the previously recommended toy balloon trick
What if you placed the pipe on a gram scale closed face down. You should at least see a force of a few grams on the scale right?

Again the balloon toy doesn’t not eliminate the possibility of the exhaust pushing off the atmosphere
 
  • #32
Chuzzled
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You seem to be complicating the scenario unnecessarily. Close off both ends of the pipe. Create a small hole on the side of the pipe near one end. Assume the environment is vacuum. What happens next?

Does air blow out? If so, what does conservation of momentum say? If not, why not?

What forces exist on the inner walls of the pipe before the air has completely been exhausted out into the vacuum. Is there an imbalance?

On a different matter. The sum of the forces on the six sides of the box is not six times the force on anyone side. Forces are vectors. They add like vectors do.
Yes the air molecules move. “Conservation of momentum is a fundamental law of physics which states that the momentumof 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.

There is no need for the pipe to apply force on the gas as pressure gradient force provides such force.

What happens when the hole is made? Gas escapes right? So when gas escapes overall pressure is reduced on all six walls. Thus no overall force.
 
  • #33
jbriggs444
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Yes the air molecules move. “Conservation of momentum is a fundamental law of physics which states that the momentumof 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).”
Quoting definitions is pointless without an understanding thereof.

The external force is provided by pressure gradient force. This force causes fluids to move from high pressure to low pressure.
This is not correct. The force that a fluid under pressure applies to the container that bounds it is not given by the pressure gradient. It is given by the pressure multiplied by the directed area on which that pressure is applied.

In the context of first year physics, pressure is a scalar and the area vector has a direction perpendicular to the surface and directed outward away from the fluid. The magnitude of the area vector is given, of course, by the surface area of the incremental element of surface area being considered. If you want to know what force a pressure exerts, you multiply the surface area vector by the pressure scalar.

The pressure gradient is given by the rate of change of pressure with respect to distance. Unlike pressure (a scalar), the pressure gradient is a vector. Its direction is the direction in which pressure rises most rapidly. Its magnitude is the rate of change of pressure in that direction. The pressure gradient determines the direction and direction and rapidity with which a small fluid element will accelerate -- local pressure gradient divided by local fluid density [and multiplied by -1].

Terms such as "field", "potential" and "gradient" are encountered in a study of vector calculus.

Again, the pipe which bounds the fluid volume does not care about pressure gradients. What happens in the middle of the fluid does not matter to the pipe. What matters to the pipe is the pressure of the fluid where it meets the pipe.
There is no need for the pipe to apply force on the gas as pressure gradient force provides such force.
This is not correct. The pressure gradient amounts to a system of internal forces within the fluid. Internal forces cannot provide any net force on the fluid.
What happens when the hole is made? Gas escapes right? So when gas escapes overall pressure is reduced on all six walls. Thus no overall force.
This is not correct. If the fluid moved in a particular direction, it gained momentum in that direction. If it gained momentum in that direction, something else must have gained momentum in the opposite direction. In the absence of pressure, none of that could have happened. But it did happen. Pressure was responsible. Pressure existed and was responsible for a non-zero net force.
 
  • #34
gmax137
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What happens when the hole is made? Gas escapes right? So when gas escapes overall pressure is reduced on all six walls. Thus no overall force.
Please read @Tom.G 's post #27, about the area of the dime-sized hole. This is the key to understanding this thing.
 
  • #35
Chuzzled
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Quoting definitions is pointless without an understanding thereof.


This is not correct. The force that a fluid under pressure applies to the container that bounds it is not given by the pressure gradient. It is given by the pressure multiplied by the directed area on which that pressure is applied.

In the context of first year physics, pressure is a scalar and the area vector has a direction perpendicular to the surface and directed outward away from the fluid. The magnitude of the area vector is given, of course, by the surface area of the incremental element of surface area being considered. If you want to know what force a pressure exerts, you multiply the surface area vector by the pressure scalar.

The pressure gradient is given by the rate of change of pressure with respect to distance. Unlike pressure (a scalar), the pressure gradient is a vector. Its direction is the direction in which pressure rises most rapidly. Its magnitude is the rate of change of pressure in that direction. The pressure gradient determines the direction and direction and rapidity with which a small fluid element will accelerate -- local pressure gradient divided by local fluid density [and multiplied by -1].

Terms such as "field", "potential" and "gradient" are encountered in a study of vector calculus.

Again, the pipe which bounds the fluid volume does not care about pressure gradients. What happens in the middle of the fluid does not matter to the pipe. What matters to the pipe is the pressure of the fluid where it meets the pipe.

This is not correct. The pressure gradient amounts to a system of internal forces within the fluid. Internal forces cannot provide any net force on the fluid.

This is not correct. If the fluid moved in a particular direction, it gained momentum in that direction. If it gained momentum in that direction, something else must have gained momentum in the opposite direction. In the absence of pressure, none of that could have happened. But it did happen. Pressure was responsible. Pressure existed and was responsible for a non-zero net force.
I didn't say the force applied on the walls was pressure gradient. Force is equal to pressure times surface area. I stated that as soon as you open the hole pressure goes down. If pressure goes down force must also go down. That's math.

Yes pressure gradient force is a vector. I am not denying that. Still vector forces can be applied to move things. The vector in this case is that high pressure moves to the direction of low pressure which is space.

Pressure gradient force is a potential force. Like buoyancy. You push down a less dense object into water giving potential energy. You let go and the object moves upwards but does not apply a force in the opposing direction.
 

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