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I'm sorry if the question is trivial but for me it is not. I'm wondering what's the principle behind the suction process in a piston pump or in a piston of an internal combustion engine. I mean when the piston goes down, pressure inside lowers and air/water enters. Is it the conservation of momentum?

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    $\begingroup$ Which part of what you stated do you not understand? You seem to have correctly stated how suction works (i.e. change in pressure). $\endgroup$ – hazzey Nov 22 '16 at 15:23
  • $\begingroup$ I was not sure of its correctness. Hence, when the piston goes down air is allowed to expand and pressure inside lowers, therefore the gas accelerate and enters into the cylinder. But what about water which is an (almost) uncompressible medium? How can it draw more water inside? Hope to be clear $\endgroup$ – horowitz Nov 22 '16 at 15:37
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    $\begingroup$ Same thing happens in both cases. Think of it this way what would happen if no water or air existed? You would create an vaccum. If you now open a valve water rushes in because the atmosphere/hydrostatic pressure pushes it in. When you have water inside the cylinder exactly the same thing happens pressure drops sucking in more water. Thus pressure generates a force. $\endgroup$ – joojaa Nov 22 '16 at 15:48
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The ideal gas law says:

$$ PV = nRT \\ $$

where $P$ is pressure, $V$ is volume, $n$ is the number of moles (molecules, basically) of gas, $R$ is the ideal gas constant, and $T$ is the absolute temperature.

If the number of molecules in the piston doesn't change and you assume that temperature stays relatively constant, then, because the ideal gas constant is a constant, everything on the right hand side of that equation is constant.

This means that pressure and volume are linearly inversely related -

$$ P = k\frac{1}{V} \\ $$

Where $k$ is a constant that affects how much of a change in pressure occurs for a particular change in volume. This is just an "umbrella" to capture the other terms that we know (are assuming) are constant. That is, $k = nRT$.

So you can see that, if you make the volume bigger, the reciprocal of volume, $\frac{1}{V}$ is getting smaller, which means the pressure $P$ is getting smaller.

Volume gets bigger, pressure gets smaller, and vice-versa.

So, when you make the volume bigger, the pressure drops, and this means that the cylinder "sucks" more fluid into the chamber.

I say fluid - a gas or liquid - but the ideal gas law is for (not surprisingly): gas. There's a similar principle at work for liquids, but you have to make some more assumptions.

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The particles ( atoms or molecules) of any fluid(liquid or gas) in a container will be in continuous random motion.During that motion they collide on the container walls and their velocity (direction or magnitude or both)changes.By momentum conservation the container wall experiences pressure.So if you move the container walls, the particles travel straight untill they hit the walls in the new position thus they fill the additional volume or at least this how my teacher explained pressure.

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    $\begingroup$ Have you included the incoming air? What about the conservation of momentum as asked? $\endgroup$ – Solar Mike Jan 7 at 7:03

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