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Homework Help: Conservation of momentum and frozen pond

  1. Dec 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A 730-N man stands in the middle of a frozen pond of radius 5m. He is unable to get to the shore of the pond because there's no friction between his shoes and the ice. So the man throws his 1.2kg textbook north horizontally at velocity of 5m/s. How long does it take him to reach the south shore?

    2. Relevant equations
    p=mv I'd presume

    3. The attempt at a solution
    so, the textbook's momentum is p=1.2(5) p=6kg m/s
    I got Pstart=Pend
    then Pstart of textbook + Pstart of man= Pend of textbook + Pend of man
    therefore, 6+73(0?)=6+73(Vf)
    then this means Vf of man is zero? I got confused after that...
  2. jcsd
  3. Dec 3, 2008 #2
    Initially, the textbook also has zero velocity.
  4. Dec 3, 2008 #3


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    The momentum of the thrown book is equal to and opposite to the momentum of the man after he throws the book. What's his velocity after he throws the book?
  5. Dec 3, 2008 #4
    You have the right equation. Just apply conservation of momentum comparing the initial conditions, which are known, to the final conditions. The book's final velocity and mass are known, and so is the man's mass, so this should allow you to find his velocity. (the only unknown in the equation)
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