# Confused about why atmospheric pressure won't move piston

• dmetrd
In summary, the conversation discusses the forces acting on a piston inside a housing and how a vacuum in the central cavity can affect its movement. The conclusion is that the pressure inside and outside the cavity must be balanced for the piston to remain stationary, and the volume of the evacuated region will decrease as the piston slides. It is also mentioned that a vacuum in a cylinder with a piston would cause the piston to move, and that reducing the volume of a vacuum is energetically favorable.
dmetrd
I'm feeling pretty silly not being able to understand this. Let's say that you have a piston (red) sitting inside of a housing (black) like this

Let's say that something like this was lying on its side on a flat table. By my understanding, if the contact between the piston and the housing is frictionless and all of the air is sucked out of the central cavity, then the only forces acting on the red piston come from the atmospheric pressure acting on the two surfaces. Since atmospheric pressure acts on a greater surface area on the left compared to the right, it would seem that there would be a greater force acting on the left than on the right, and the piston would slide to the right. This result seems completely inane, but I can't for the life of me justify the expectation of nothing moving using a statics force balance. Could someone help me out with this?

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dmetrd said:
it would seem that there would be a greater force acting on the left than on the right, and the piston would slide to the right
That is correct.

CWatters, russ_watters and Chestermiller
Consider this: What happen to the volume of the evacuated region as the piston slides left to right?

Yet another way to look at this ...

Suppose the two piston are not connected, and you are able to keep them form moving by applying an external force to each (in the outward direction of course). Which piston will require the greater force ?

CWatters
Dale said:
That is correct.
Hm, I guess that makes sense, but it doesn't seem intuitive to me. Is there anything about this that would make this impossible to recreate in the real world? Even without creating a vacuum in the center cavity, the area on the inside of the cavity is smaller than on the outside due to the connecting rod so it wouldn't balance the force.

Janus said:
Consider this: What happen to the volume of the evacuated region as the piston slides left to right?
The volume of the evacuated region would decrease, but would that affect anything if there was no air in the cavity?
SammyS said:
Yet another way to look at this ...

Suppose the two piston are not connected, and you are able to keep them form moving by applying an external force to each (in the outward direction of course). Which piston will require the greater force ?

With no rod between the pistons, I suppose the larger one would require more external force.

I'm not sure what to take from all of this. So it seems like I never actually described anything that isn't correct?

CWatters
dmetrd said:
Even without creating a vacuum in the center cavity, the area on the inside of the cavity is smaller than on the outside due to the connecting rod so it wouldn't balance the force.
If the pressure inside the cavity is the same as the external pressure then the forces are completely balanced. The area of all the "left-facing" surface is inherently always equal to the area of all the "right facing" surface.

Janus said:
Consider this: What happen to the volume of the evacuated region as the piston slides left to right?
It would decrease. I am curious what you are leading to, though.

DocZaius said:
It would decrease. I am curious what you are leading to, though.
If you put a vacuum in a cylinder with a piston, it sucks the piston in. More generally, if a mechanism can reduce the volume of a vacuum then, all other things being equal, it is energetically favorable for it to do so.

## 1. Why won't atmospheric pressure move the piston?

Atmospheric pressure alone is not strong enough to move the piston. It needs an external force, such as the pressure of a gas or liquid, to create a greater force on one side of the piston and push it in the desired direction.

## 2. How does atmospheric pressure affect the movement of the piston?

Atmospheric pressure is the force exerted by the weight of the air above a certain area. It creates a downward force on the piston, but unless there is an unequal force on the other side, the piston will not move.

## 3. Can atmospheric pressure be used to power a piston?

Technically, yes, but it would require a significant difference in pressure on either side of the piston. This can be achieved with advanced engineering techniques, but it is not a practical means of powering a piston.

## 4. What other factors besides atmospheric pressure can affect the movement of a piston?

The design and weight of the piston, the amount of friction in the mechanism, and the temperature can all impact the movement of a piston. Additionally, any external forces, such as gas or liquid pressure, can also affect the piston's movement.

## 5. How is atmospheric pressure measured?

Atmospheric pressure is commonly measured with a barometer, which uses a column of liquid (usually mercury) to indicate the pressure. It is also measured in units called millibars or pounds per square inch (psi).

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