# In a cylinder, why do we use just the ground area to get p?

1. Oct 18, 2016

### MicroCosmos

Hi,

given an Hydraulic Cylinder with the Formula:
F=p*A

Why do we use APiston to calculate the Force in Work-Direction? Doesnt it suppose the "Potential Energy" of the compressed air just presses in that Area?

Im pretty confused, sorry about the unconcrete question.

2. Oct 18, 2016

### Grinkle

The potential energy of the compressed air is what causes the p in your equation. Can you re-state your question? I am not sure what you are asking.

3. Oct 18, 2016

### MicroCosmos

The fact is that a Zylinder is built with 2 circles and 1 rectangle. But when i look for the formula that calculates the Force applied to one of the circles (Like Piston with compressed fluid work) depending on the Pressure, i find everywhere: F=p*ACircle, where A is the Area of that circle. My question is: isnt it wrong? because the pressure schould cause Force in all surfaces, it should distribute among all of them, causing an smaller Force than F=p*ACircle tells me.

4. Oct 18, 2016

### Grinkle

There is a force pushing against the piston surface that is p(area of the piston surface).

There is a force pushing against the wall of the cylinder that is p(area of the cylinder wall).

It is not an either / or, the pressure presents a force against all surfaces of the interior of the cylinder, and the force against any part of that interior surface is the pressure times the area of interest.

Work is force * displacement, so when calculating work, the relevant area to use is the piston surface, because that is the surface that is involved in doing work, because it is displacing against a force.

As the piston moves up, the p decreases, of course, since the volume of the interior of the cylinder is increasing. This is where you are getting your spring analogy from, I think.

5. Oct 18, 2016

### MicroCosmos

I understand what you mean, i was confused, thanks a lot. I thought the fact that a Force acts in a surface means there is less pressure for the others to hold. That is of course not true.