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Homework Help: Momentum of System

  1. Nov 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A system initially consists of a ball of mass m at rest at a height h above the Earth and the Earth, of mass Me. When the ball has fallen a distance h/2, the total momentum of
    the system is
    A. 0
    B. m(sqrrt)2gh
    C. Me(sqrrt)2gh
    D. Me + m(sqrrt)2gh
    E. Me - m(sqrrt)2gh


    2. Relevant equations



    3. The attempt at a solution
    I put B and missed it. I think the answer is C but not sure.
    Any help understanding this would be appreciated.
    Thanks,
    Kevin
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 9, 2008 #2

    [tex]\textbf{v}_f^2=\textbf{v}_i^2+2\textbf{ad}[/tex]

    so, [tex]\textbf{v}=\sqrt{2\textbf{ad}}[/tex], and:

    [tex]\textbf{p}=m\textbf{v}=m\sqrt{2\textbf{ad}}[/tex].

    The total momentum of the system is the sum of momenta of the particles. The above momentum is that of the ball. The momentum of the earth is given by:

    [tex]M_e\textbf{v_e}=[/tex],

    add them and that's your answer which isn't readily available, sadly. Maybe someone else has more of an intuitive approach.
     
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