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Homework Help: Biomechanics - Momentum/Collisions

  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 (meaning going in an opposite direction). 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 2, 2007 #2


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