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Jar with an object

  1. Jul 22, 2007 #1
    A jar filled partially with water has an object floating in it. The jar is open. Now, if the jar is closed and a vacuum pump is used to pump the air out, what will happen to the floating object? will it rise up more or sink or ...? and why?

    What do you think of the following approach :

    Normally, It would be impossible to pump all the air out but if you theoretically did, (assuming that the jar doesn't shrink) you would be forcing the water to occupy a large space thus turning it into water vapour. (similarly, if you add pressure to a tank of propane (vapour), it turns into liquid due to pressure.

    So if it turns into gas, the object will no longer float.

    But thinking more logically, If the object floats in water, that would mean that there is either air trapped in the object or the object is less dense than the water.

    If there is air, the vacuum would remove it, thus expanding (more space between the particles) the object at the same rate as the water.

    So assuming that the object remains intact (not expanded to the point that the molecules separate), the object will stay in the same position and neither rise or fall.
     
  2. jcsd
  3. Jul 22, 2007 #2

    cristo

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    If you sucked the air out of the jar, then the density of the water would decrease. Thus the object would be more dense than the water. I conjecture that the object would sink. (I'm not saying I'm right; that's why it's a conjecture!)
     
  4. Jul 22, 2007 #3
    Cristo, you assume the object will not change in density (and that its density was very close to that of water, to begin with). If it were a marshmallow..
     
  5. Jul 22, 2007 #4

    cristo

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    That's a very good point! I just went with my gut on that one, which I probably shouldn't do in future! :blushing: However, if the object was pretty much rigid, and was only just floating, then my guess wasn't far off!
     
  6. Jul 22, 2007 #5

    Danger

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    I'm with Cristo on this one. The OP didn't specify that the floating object was elastic. If it isn't, and there's no reason to think that it is, then it will retain its original density which will be greater than that of the expanded liquid.
     
  7. Jul 23, 2007 #6
    I reckon the typical object will continue floating, since water isn't very compressible. (Of course, the water level will rise slightly, and I'd naively expect the object to also rise slightly, just less than the water level.)
     
    Last edited: Jul 23, 2007
  8. Jul 23, 2007 #7

    Danger

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    I didn't think of this earlier, but I now believe that it would depend upon how much water there is and how large the container is. The first time around, I was thinking that all of the liquid would expand to fill the available volume. Now I'm inclined to believe that there'll be liquid on the bottom with vapour above. If that's the case, then the object will float on the liquid surface.
     
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