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Homework Help: Momentum Question

  1. Oct 22, 2007 #1
    1. The problem statement, all variables and given/known data
    A powerful, 100-kg man is kneeling in the middle of a frozen lake, holding two large rocks. One rock has a mass of 25 kg, and the other a mass of 15 kg. The man wants to quickly get to the south edge of the lake, and he knows that he needs to throw his rocks to the north to propel himself there. He always throws each rock away from himself at 20 m/s.

    What will be his final speed if he
    a) throws both rocks simultaneously?
    b) throws the heavy rock and then the lighter rock?
    c) throws the lighter rock and then the heavy rock?

    2. Relevant equations
    m[tex]\Delta[/tex]v[tex]_{initial}[/tex] = m[tex]\Delta[/tex]v[tex]_{final}[/tex]

    3. The attempt at a solution


    m[tex]_{man}[/tex]v = (m[tex]_{rock1}[/tex]+m[tex]_{rock2}[/tex])v[tex]_{rock}[/tex]

    100v = (20+15)20
    v = 8 m/s


    mv = m[tex]_{rock1}[/tex]v[tex]_{rock}[/tex]
    100v = 25*20
    v = 5

    mv[tex]_{final}[/tex]-(m+m[tex]_{rock2}[/tex])v[tex]_{initial}[/tex] = m[tex]_{rock2}[/tex]v
    100v[tex]_{final}[/tex]-(100+15)*5 = 15*20
    v[tex]_{final}[/tex] = 8.75 m/s

    (c) I get the same answer as (b), using the same methods, which is quite weird.

    The correct answers are 5.71 m/s for (a), 6.18 m/s for (b), and 6.14 m/s for (c). I'm not sure what I'm doing wrong for each part...
  2. jcsd
  3. Oct 22, 2007 #2
    Your solution for a) is correct. In solution for b) when the heavier rock is thrown the man is still holding the other rock. So his net weight will be 115 kg. The second rock is thrown with a speed of 20 m/s relative to the man, in the direction opposite to in which the man is moving after throwing the first rock. Its speed relative to the ground will be (20 - speed of the man after throwing the first rock) in the opposite direction. Solve on the above basis. Answers given appear to be incorrect.
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