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Homework Help: Relativistic momentum football problem

  1. May 10, 2009 #1
    1. The problem statement, all variables and given/known data

    A football player with a mass of 82.1 kg and a speed of 2.00 m/s collides head-on with a player from the opposing team whose mass is 126 kg. The players stick together and are at rest after the collision. Calculate the speed of the second player, assuming the speed of light is 3.00 m/s

    2. Relevant equations
    p= mv/[sqrt(1-v^2/c^2)]

    3. The attempt at a solution
    Basically how i attempted to solve this is used the above equation for each player, and set them equal to each other using 3.00m/s as the speed of light, because if they are at rest after the collision then their momentum must be equal. Because i have knowns for 1 player, and the mass of the other player, I should be able to solve for v, the other players speed, but the answer i calculate isn't right.... any suggestions?
  2. jcsd
  3. May 10, 2009 #2


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    Homework Helper

    First suggestion: show your work in more detail... I don't notice any obvious issues but maybe if you actually specify what you did something will become clear.
  4. May 10, 2009 #3
    p= 82.1*2/[sqrt(1-2^2/3^2)]
    p= 220.297

    v= 1.94m/s which is not correct
  5. May 11, 2009 #4
    Check your answer by putting v=1.94 in the right hand side
    (I get 320).
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