Because water is non-compressible, a container filled with water doesn't react like a container filled with air would (e.g. deforming in response to impacts etc.), or even the way a container half-filled would. So, does it act the same as a solid?

I don't know any physics, so please explain like I'm an Arts major.

  • $\begingroup$ Water (and other liquids and solids) are compressible, just not as much as gases. A liquid-filled object will not behave exactly as a completely solid object because of the possibility of dissipation in the liquid. $\endgroup$ – Jon Custer Aug 18 '17 at 15:49
  • $\begingroup$ @JonCuster Mechanical dissipation will also occur in the solid due to internal friction. However, at least two other things will differ between containers filled with a solid vs. a liquid: the liquid will not resist a shear load on the container, and natural convection will occur in the liquid if a temperature gradient (and a gravitational field) exists. But in any case, I agree that the premise of the question is incorrect because water (and all liquids and solids) are compressible, as quantified by the bulk modulus. $\endgroup$ – Chemomechanics Aug 18 '17 at 16:02
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    $\begingroup$ Imagine a plastic bag filled with liquid. Does the behave like a solid? Imagine the plastic bag half-filled with liquid. Same question. $\endgroup$ – Chet Miller Aug 18 '17 at 16:25
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    $\begingroup$ There will also be the following difference: if you rotate the bottle along its axis, the water inside will also rotate. If you stop the rotation, the water will still move for a while since its adherence to the bottle is weak. This woudl not happen with a solid object: in this case every part of the object will stop, but you'll need more force to stop it. $\endgroup$ – EigenDavid Aug 21 '17 at 9:25
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    $\begingroup$ @Benubird - Take two bottles completely full of water, one frozen and one liquid. Place them on a flat surface and then lower a flat plate with a significant amount of mass onto each bottle. Once at rest, move the top plate parallel to the flat surface on which the bottles rest. The bottle with the liquid water will likely rip apart and explode while the ice-filled one will slide against one of the plates (assuming not too much mass on top plate). $\endgroup$ – honeste_vivere Aug 21 '17 at 14:31

Water isn't completely incompressible but to compress it you need so much pressure that often we approximate it to be incompressible.

Having said that there are still a few differences. The incompressibility of water makes it resistant to changes in volume but it can still change in shape. If you fill a plastic water bottle completely with water you can't really change it's volume. I just tried it and the water bottle could easily hold my weight. It flattened a little because the easily deformable bottle could become flatter without changing its volume (although there is no easy way to test if the volume really stayed the same). You could counteract this by increasing the pressure of the water but this is cheating because a high pressure gas also acts more like a solid. A soccer ball that's under high pressure also almost acts like a solid.

Another difference is that water can move freely. A fully filled water bottle can spin more easily around its axis than a bottle filled with a solid of the same density. This is because initially the walls of the bottle can spin without the water spinning along with it. Generally the water can have all sorts of crazy currents inside which can affect the rotation of the bottle. These currents don't influence the center of mass though because the density of the fluid is constant. When there is air inside sloshing can happen and this forms a big problem for trucks carrying liquid or for spacecraft. The sloshing makes braking harder and also makes it harder to maneuver and that's why there is a lot of research in slosh dynamics.

So in conclusion water in a closed container behaves a little like a solid but it is still fundamentally different. It is resistant to changes in volume but can still change shape (think of water balloons). Because it is completely filled it shows no sloshing but it still behaves differently because the fluid is able to move freely inside.

Oops I just noticed this post is over 3 years old.

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