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Homework Help: Conservation of Momentum (1-D)

  1. Jan 23, 2013 #1
    1. The problem statement, all variables and given/known data

    Football player 1 with a mass of 110kg is moving at 8.0m/s when he crashed into football player 2 with a mass of 105kg, moving at 12m/s [N]. During the tackle, football player 1 hangs onto football player 2, giving them the same velocity during the 0.3 second collision.

    A) What is their velocity after the collision?
    B) What is the net force (affecting each player) during the collision?

    2. Relevant equations

    p = momentum
    v' = velocity after collision


    Net Force = Δp / Δt

    Conservation of momentum:
    m1v1 + m2v2 = m(v')

    3. The attempt at a solution

    m1 = 110kg
    v1 = -8m/s

    m2 = 105kg
    v2 = 12m/s

    Let [N] be positive.


    m1v1 + m2v2 = m(v')
    110kg(-8m/s) + 105kg(12m/s) = (110kg+105kg)v'
    -880kg(m/s) + 1260kg(m/s) = 215kg v'
    380kg(m/s) / 215kg = v'
    v' = 1.76744186 m/s [N]

    I've verified this answer with the course's book. I'm including it because the answer for part B depends on the answer from part A.


    Net Force = Δp / Δt
    Net Force = (m1v' - m1v1) / (0.3 seconds)
    Net Force = [(110kg(1.76744186m/s)) - (110kg)(-8m/s))] / (0.3 seconds)
    Net Force = [ 194.4186046kg(m/s) + 880kg(m/s) ] / (0.3 seconds)
    Net Force = (1074.4418605kg(m/s)) / (0.3 seconds)
    Net Force = 3581.395349kg(m/s2)

    Net Force on Player 1 = 3581.395 Newtons
    Net Force on Player 2 (from Newton's 3rd LoM) = -3581.395 Newtons

    I think this should be correct, but I think I'm doing something wrong. The book's answer work uses the formula :
    Net Force = (m1v1 - m1v' / (0.3 seconds)
    for net force, which doesn't make sense to me... seeing as how a change in momentum should be found by subtracting the final value from the initial.

    By the way, the book's answer for player 1 is :
    -2280 N

    .... Which makes sense if you use their formula (the calculations are correct)... but its the formula itself that I don't understand.

    Any thoughts?
  2. jcsd
  3. Jan 23, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You are correct. The book makes two errors:
    (1) The formula is wrong; Change in anything is always final minus initial, as you point out.

    The second error the book makes, which is much worse, is:
    (2) They neglect the sign of the velocities. That's the only way they can get this answer.

    If all they did was use initial minus final, then there answer would be the same as yours except for a sign. No big deal, since the force acts on each player in different directions anyway.
  4. Jan 23, 2013 #3
    Here's what they did:

    For player 1:

    FNet (Δt) = mv2 - mv1
    FNet (0.3) = (110)(-8) - (110)(+1.77)
    FNet (0.3) = -685.3
    FNet = -2280

    The actual calculation is correct, but the formula isn't... I'm not sure what they're playing at.
  5. Jan 23, 2013 #4

    Doc Al

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    Staff: Mentor

    The calculation is OK up to here.
    But this step is wrong. (Do it yourself and see!)
    The calculation is not correct.
  6. Jan 23, 2013 #5
    Yeah my bad, that's very wrong.
    Shouldn't accept their answers at face-value, lol.

    So my initial answer was correct?
  7. Jan 23, 2013 #6

    Doc Al

    User Avatar

    Staff: Mentor

  8. Jan 23, 2013 #7
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