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Homework Help: Elastic Collision

  1. Mar 1, 2007 #1
    1. The problem statement, all variables and given/known data
    A collision occurs between two football players. Player 1 has mass of 75 kg and a velocity of 6 m/s. Player 2 has mass of 150 kg and a velocity of -3 m/s. Assuming an ELASTIC collision, what is the final velocity of player 2?

    2. Relevant equations
    They gave us this equation in class to calculate the final velocity:
    m = mass
    Vi = initial velocity
    Vf = final velocity
    a = player 1
    b = player 2

    Vfb = [(2*ma)/(ma+mb)]*Via + [(mb-ma)/(ma+mb)]*Vib

    3. The attempt at a solution
    Using the equation above and plugging in the numbers:

    Vfb =
    [(2*75kg)/(75kg + 150kg)]*(6 m/s)
    [(150kg - 75kg)/(75kg + 150kg)]*(-3 m/s)

    which gives me:

    Vfb = 4 m/s + -1 m/s = 3 m/s

    The "online" homework solution says it is -3 m/s. Am I incorrect? Did I miss a negative sign somewhere, or is there a principle of momentum that I am not getting?
  2. jcsd
  3. Mar 1, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    That answer implies that the velocity of the 2nd player is unchanged by the collision--which makes no sense.
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