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Whenever energy is added to a system, the system gains mass

  1. Jan 27, 2009 #1

    dav2008

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    Gold Member

    "Whenever energy is added to a system, the system gains mass"

    http://en.wikipedia.org/wiki/Mass-energy_equivalence#Practical_examples
    From wikipedia:
    Is this actually true? If someone said this to me in a conversation I would tell them compressing the spring does absolutely nothing to its mass and increasing the temperature of an object does nothing to its mass.

    Now it's on Wikipedia so I'm wondering if I'm missing some concept here...

    I was going to remove it but I wanted confirmation first.
     
  2. jcsd
  3. Jan 27, 2009 #2

    Nabeshin

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    Re: "Whenever energy is added to a system, the system gains mass"

    The temperature example sounds plausible, but I don't know about the compressed spring. By that same token of logic, if PE increases mass, lifting a box will increase the mass?

    Temperature I can get because of the motion of the particles etc. but nothing fundamental changes about a box when I lift it 1m, giving it gravitational potential energy.
     
  4. Jan 27, 2009 #3

    jtbell

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    Staff: Mentor

    Re: "Whenever energy is added to a system, the system gains mass"

    I gave an answer to this question in the following thread just a few minutes ago:

    https://www.physicsforums.com/showthread.php?t=287666
     
  5. Jan 27, 2009 #4
    Re: "Whenever energy is added to a system, the system gains mass"

    dav, did you bother to read the reference ("[8]") for the paragraph you wish to delete (from the wikipedia page on a topic that you don't understand)?

    Yes, compressing a spring or heating a potato increase their mass. (Hint: in one sense we can say that energy is mass, and nobody disagrees with this when we are referring to a system that is at rest.)

    Lifting a box higher above the earth can increase the mass of a system encompassing both the earth and the box, but not of the box itself (as Nabeshin and jtbell noted).
     
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