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Homework Help: Another momentum problem!

  1. Aug 15, 2010 #1
    1. The problem statement, all variables and given/known data

    A 92 kg fullback, running at 5.0 m/s, attempts to dive across the goal line for a touchdown. Just as he reaches the goal line, he is met head-on in midair by two 75 kg linebackers, one moving at 2.0 m/s and the other at 4.0 m/s. If they all become entangled as one mass, with what velocity do they travel? Does the fullback score?

    2. Relevant equations

    Momentum = Mass x velocity
    P stands for Momentum
    Pbefore = Pafter

    3. The attempt at a solution

    Here is what iv'e done so far. I want to make sure its right.

    First I added both of the momentums of the linebackers and then I subtracted that momentum from the fullbacker's momentum to get a difference of 10 kg m/s and then divided that by the total mass of the 3 players combined.

    P = M xv
    P = Mv1 + Mv2
    P = (75)(2) + (75)(4)
    P = 150 + 300
    P = 450 kg m/s
    Pfullback = 92 x 5 = 460 kg m/s
    Pchange = 460 kg m/s – 450 kg m/s
    Pchange = 10 kg m/s
    totalVelocity = totalMomentum/totalMass
    Velocity = 10 kg m/s / 242kg
    Velocity = 0.041 m/s
    Yes the fullback scores.
  2. jcsd
  3. Aug 15, 2010 #2
    Good work. Plus I like your enthusiasm for the physics of football.
    But how many significant figures should your final answer have?
  4. Aug 16, 2010 #3
    I actually don't know how many sig figs. He tells me to watch my sig figs but I just round whenever it seems necessary :p thanks for confirming :)
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