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Particle momentum problem

  1. Mar 11, 2006 #1
    6. Two particles, each of mass m, are connected by a light inextensible string of length 2l. Initially they lie on a smooth horizontal table at points A and B distant l apart. The particle at A is projected across the table with velocity u. Find the speed with which the second particle begins to move if the directions of u is.
    (a) along AB,

    mu = (m+m)v
    v = u/2

    But the given answer is u/2 and mu/2. Can anyone explain to me why there are 2 answers? Thanks.
     
  2. jcsd
  3. Mar 11, 2006 #2

    Doc Al

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    Only the first answer is a speed; the other is a momentum.
     
  4. Mar 11, 2006 #3

    Hootenanny

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    I can't see the answer of [itex]\frac{1}{2}mu[/itex] being correct, because that would leave the velocity having units of [itex]m\cdot kg \cdot s^{-1}[/itex], which doesn't happen.:yuck:
     
  5. Mar 11, 2006 #4
    Thanks..I am so stupid...
     
  6. Mar 11, 2006 #5

    Hootenanny

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    It really is bad practise by a book to ask for a velocity then give an answer in momentum. I've just had some problems with a textbook myself ...:mad:
     
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