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How can two objects colliding and merging have a bigger mass?

  1. Jul 18, 2009 #1
    i've read that the mass of two objects which is formed when two equal objects collide must be twice the mass of the moving objects, which is grater then the sum of their rest mass. does it realy happen in terms of particals? how is it explained?

  2. jcsd
  3. Jul 18, 2009 #2


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    I think that the mass of the final object depends on the relative speed of the two objects before colliding, if both have the same rest mass.
  4. Jul 18, 2009 #3
    That is right......the extra kinetic energy must have gone somewhere...we might say that it is converted to heat or some other form of internal energy..but since energy equals mass, this extra energy will give a greater mass to the final system..thus it is also true that you can make a body more massive by just raising its temperature...Please refer the book on special relativity by Resnick...section 3.6..
  5. Jul 18, 2009 #4
    thank you very much.
    does a body whose mass has been increased by high temprature acts the same as though he was simply that massive? how hot must something be in order for the added mass not to be neglectable?
  6. Jul 18, 2009 #5


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    Heat isn't the only kind of energy. [math]E= mc^2[/math] so [math]m= E/c^2[/math]. How large that has to be to be "not neglectable" depends upon what you consider "negletable".
  7. Jul 19, 2009 #6
    i thought that the energy from e=mc^2 is simply the energy that can be gained by using the mass, is that wrong?
  8. Jul 19, 2009 #7


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    Well, it depends on what you mean by "using". [itex]E = mc^2[/itex] is the amount of energy you'd get if you completely converted the mass into energy. For instance, by reacting matter and antimatter. If that's what you mean by "using", then sure, [itex]E = mc^2[/itex] is the amount of energy you can get. But a normal definition of "using" probably entails something like nuclear fusion, or burning - some process in which there's still some mass left at the end - and in that case you only get a small fraction of [itex]mc^2[/itex] as useful energy.
  9. Jul 19, 2009 #8


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    It works both ways: mass contains energy, energy has mass.
  10. Jul 20, 2009 #9
    i think i've got it now, thank you.
  11. Jul 21, 2009 #10
    And if you want to do a small calculation to see how temperature can affect mass, just equate mc^2 and kT, where k is the Boltzmann constant and T is the temperature,,,,and calculate T for some amount of mass..
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