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Homework Help: A rock from a boat - could you help me understand?

  1. Oct 19, 2012 #1
    1. The problem statement, all variables and given/known data

    A man on a boat throws behind him a rock of mass m1=2kg with a velocity of v1=18m/s. The mass of a man with a boat is m_2=90kg. What velocty v_2 will the boat go with?

    2. Relevant equations

    v2 = (m1*v1)/m2

    3. The attempt at a solution

    So I was upsent on the lesson where we were speaking about this and can't figure out where does the formula I wrote above come from. Why is it certain that m2v2=m1v1? Of course I know I can just plug the numbers inside and bam, I've got my answer but I'd rather know why does it work. Could you help?
  2. jcsd
  3. Oct 19, 2012 #2
    Well the eqaution implies that: mom boat - mom rock = 0
    Why would that be?
  4. Oct 19, 2012 #3

    Doc Al

    User Avatar

    Staff: Mentor

    It comes from the conservation of momentum. Since (presumably), the man and boat start out at rest, the total momentum is zero.

    You might want to read this: Momentum Conservation in Explosions
  5. Oct 19, 2012 #4
    Oh, I see. Thank you a lot!
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