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Homework Help: Relativistic momentum and conservation of this

  1. Oct 30, 2007 #1
    1. The problem statement, all variables and given/known data
    A particle with mass m and speed (4/5)c collides with a particle (with mass m as well, but speed = 0) un-elastically, and continues as one particle.

    I have to find the speed and mass for this "joined" particle.

    3. The attempt at a solution

    Ok, first - conservation of momentum!

    1) I use that gamma_1*m*(4/5)c + 0 = gamma_2*M*v

    2) I also use gamma_1*m + m = gamma_2*M

    Is this correct? In the notes I took in class today, I wrote this for #1: gamma_1*m*(4/5)c + 0 = gamma_2*M - but I guess I wrote this one down wrong?

    Thank for you all in advance.
    Last edited: Oct 30, 2007
  2. jcsd
  3. Oct 30, 2007 #2


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    Your equations 1 and 2 are correct. Note that gamma_1 is known so that leaves two unknowns: M and v.
  4. Oct 30, 2007 #3
    Great, thanks!
  5. Oct 30, 2007 #4


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    M=2m. Then gamma2 cancels out of the equation for v,
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