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Relativistic Collision

  1. Nov 21, 2009 #1
    To a moderator: this is a theoretical, concept-based question.

    Say two balls of putty, moving relativistically near the speed of light, collide (although i understand this is not possible theoretically and realistically). They collide at a slight perpendicular displacement, instead of head-on, so that in the final state the stuck together system is rotating. Compared to the non-rotating head-on collision, how will this effect the final speed, and how will this effect the Mf' (final mass)? Imagine that you stop the lump from spinning, will its mass be great. less, or equal to Mf'?

    Here is what I got out of this situation; Vf remains the same, since rotation does not affect the conservation of momentum. However, less "energy" goes into the mass conversion, so there is less mass. If we stop it from spinning, we do work on the system, adding this rotational energy into the system and the mass willl then be greater to the final mass.

    I'm not sure if I'm understanding the theory correct.

    P.S. Please don't say "it will vaporize" or "this is not possible"; Let's ASSUME that they stick together relativistically and also rotate, without vaporizing.
  2. jcsd
  3. Nov 21, 2009 #2
    If two colliding bodies attach to each other, the system mass is determined with their initial energies solely. It does not matter whether they rotate or not. In the frontal collision it is their atoms/molecules that will rotate/vibrate more.

    If you stop the rotation, the mass will become smaller because you take away some energy from the system.
  4. Nov 21, 2009 #3
    by looking at the equations of relativity, your answer actually does make sense; rotate or not, it will have the same final speed and thus same final mass! Thank you so much!

    Can anyone confirm this?
  5. Nov 21, 2009 #4


    Staff: Mentor

    Yes, you can always analyze the collision from the center of mass frame. In that frame if the final mass is spinning then it will have more energy and therefore more mass than an otherwise identical non-spinning mass. This energy and mass will come from the original mass energy of the system.
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