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A very stupid question

  1. Sep 12, 2009 #1
    I think its so stupid, that it doesn't deserve to be in any of the other forum sections >.>

    If I have a block of ice with air in it and measures its mass.... does the air bubbles get massed as well?
     
  2. jcsd
  3. Sep 12, 2009 #2

    LeonhardEuler

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    Gold Member

    There are different ways of interpreting that question. I'll say this: If you have a block of ice with bubbles inside, and the same amount of ice without the bubbles, a scale will record the same weight (neglecting the effects of the temperature of the ice which changes the density of air around it).

    Basically, when you weigh something, you are actually weighing the difference between the weight of the object and the weight of the air it displaces. Normally, this is insignificant since most things you weigh are much denser than air. However, if you weigh something like a helium balloon, you can even get a negative weight.

    If you put holes in something and fill it with air, then the extra air just displaces other air that has the same mass, so it has no effect. If you put holes in something and leave a vacuum, the weight you measure with a scale will be less.
     
  4. Sep 12, 2009 #3
    oh that makes sense, thnx
     
  5. Sep 13, 2009 #4
    You might want to Google Archimedes' Principle, which states that the upward force due to the air (i.e. the buoyant force) is equal to the weight of air displaced by the object. If you put bubbles in a rigid object, the object itself can't be compressed, and will thus have to displace extra air. That air will exert its own weight in force against the object.

    If there is compression of the object on a small scale though, then yes, you would be able to weigh the air.
     
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