The discussion centers on the forces acting on a closed container filled with air when it is brought into a vacuum and the lid is removed. Participants explore the implications of pressure changes, conservation of momentum, and the dynamics of air movement in relation to the container's motion. The scope includes theoretical considerations and conceptual reasoning.
Participants do not reach a consensus on whether the container moves after the lid is opened. There are multiple competing views regarding the forces acting on the container and the implications of air movement.
Participants express various assumptions about pressure, volume, and the nature of forces acting on the container, but these assumptions remain unresolved and are subject to interpretation.
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?gmax137 said: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
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).”jbriggs444 said: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.
Quoting definitions is pointless without an understanding thereof.Chuzzled said: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).”
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.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 pressure gradient amounts to a system of internal forces within the fluid. Internal forces cannot provide any net force on the fluid.There is no need for the pipe to apply force on the gas as pressure gradient force provides such 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.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.Chuzzled said: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.
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.jbriggs444 said: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.
Force is a vector. Pressure has gradients. You cannot compute total force by multiplying total surface area by average pressure.Chuzzled said: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.
A gradient is the opposite of a potential.Chuzzled said: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.
jbriggs444 said:Force is a vector. Pressure has gradients. You cannot compute total force by multiplying total surface area by average pressure.
If you reduce pressure on one side of the pipe, net force goes up, not down.
A gradient is the opposite of a potential.
The if a vector field is the "gradient" of a scalar field, then the vector field is conservative and the scalar field is the "potential" of the vector field.
How does net force go up. Force =pressure*surface area. That’s a fact. https://www.dummies.com/education/science/physics/how-to-calculate-force-based-on-pressure/jbriggs444 said:Force is a vector. Pressure has gradients. You cannot compute total force by multiplying total surface area by average pressure.
If you reduce pressure on one side of the pipe, net force goes up, not down.
A gradient is the opposite of a potential.
The if a vector field is the "gradient" of a scalar field, then the vector field is conservative and the scalar field is the "potential" of the vector field.
Force is a vector. That is a fact also. The net force from equal pressure on the six faces of a cube is zero.Chuzzled said:How does net force go up. Force =pressure*surface area. That’s a fact.
Sure if you want to put it that way. But there is still force applied to all six walls. If the walls were elastic like a balloon, the pressure force would cause the walls to stretch.jbriggs444 said:Force is a vector. That is a fact also. The net force from equal pressure on the six faces of a cube is zero.
To get the net force you find the forces on each face and add them all up as vectors.
Nope. There is an asymmetry. There is a hole in one wall. That wall is subject to less force than the rest. That means that there is an unbalanced net force. The balloon moves as a result.Chuzzled said:Sure if you want to put it that way. But there is still force applied to all six walls. If the walls were elastic like a balloon, the pressure force would cause the walls to stretch.
But as soon as air starts coming out you will notice the balloon walls contracts. If there was an uneven force, you would see it on the balloon walls. Instead you see all the walls contract evenly
How can eliminate the possibility of the exhaust pushing off the atmosphere. Do you have a simple experiment to demonstrate this?jbriggs444 said:Nope. There is an asymmetry. There is a hole in one wall. That wall is subject to less force than the rest. That means that there is an unbalanced net force. The balloon moves as a result.
The simplest experiment is hazardous. Place a compressed air cylinder on its side and break off the valve. The resulting force is larger than atmospheric pressure multiplied by area of the hole. A loose cylinder can break through walls and wreak havoc.Chuzzled said:How can eliminate the possibility of the exhaust pushing off the atmosphere. Do you have a simple experiment to demonstrate this?
Oh, we had this a few months ago - a silly youtube video of someone preventing a balloon rocket or something from moving with a vacuum cleaner. We're not going to indulge this. If you want to learn how rockets work, we can teach you, but you need to accept the teaching, not play silly games. Feel free to start a new thread, but only if you are going to accept and learn the actual physics. With diagrams and math and straightforward real world examples. No games with silly, purposely convoluted examples and no argementativeness.Chuzzled said: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?