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We all know pressure is relative, and the pressure of a containment changes with change in its surrounding pressure. Now comes the obvious question:

If I have a water balloon half filled in a container, and I vacuum the container to a certain negative pressure (e.g -600mbar), what will be the pressure by which the water balloon pushes out?

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The main concept of pressure in physics is an absolute quantity, because it gives directly net force on surface from the fluid on one side of it. This force has measurable effects and so absolute value of pressure can be in principle determined.

There is also concept of relative pressure, but used more in engineering/aviation for practical reasons (concept of pressure is used in the context of Earth's atmosphere being the standard value). If the relative pressure of atmosphere in a container has relative pressure -600mbar with respect to standard atmosphere which has absolute pressure 1000mbar, this means the gas in the container has absolute pressure 400mbar. If the balloon is in mechanical equilibrium with this gas, it pushes back on the gas with absolute pressure 400mbar.

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  • $\begingroup$ In such case where the balloon is half filled with water, if I poke the balloon, does the water rush out into the container and at what pressure is it pushing out with? $\endgroup$ – TechDroid May 14 at 15:46
  • $\begingroup$ Yes the water should go out since inside the ballon the pressure is higher than outside. Pressure of water depends on position, inside the balloon it won't change much, the water outside will have pressure of the atmosphere, i.e. 400mbar. $\endgroup$ – Ján Lalinský May 14 at 16:22
  • $\begingroup$ @ Jan. If the pressure in the balloon is greater than outside, by how much exactly? And why did you state that the absolute pressure in the balloon is also 400mbar? $\endgroup$ – TechDroid May 16 at 14:13
  • $\begingroup$ How much depends on the material the balloon is made of, its temperature and its dimensions. For the usual few liter sized balloon, the pressure inside is only small fraction of the total pressure higher than the outside pressure. I didn't state that the pressure inside the balloon is 400mbar, but that the balloon acts with pressure 400mbar on the gas outside, which has 400mbar. The gas inside will be slightly higher than 400mbar, $\endgroup$ – Ján Lalinský May 20 at 21:26

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