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Homework Help: String through hole - central forces

  1. Feb 10, 2010 #1
    1. The problem statement, all variables and given/known data

    Two particles of mass m are connected by a light inextensible string of length l. One of the particles moves on a smooth horizontal table in which there is a small hole. The string passes through the hole so that the second particle hangs vertically below the hole. Use the conservation of energy and angular momentum to show that:

    r(dot)^2 = (gl+v^2)/2 - (lv)^2/(8r^2) - gr

    where r(t) is the distance of the first particle from the hole.

    2. Relevant equations

    3. The attempt at a solution

    I tried to solve this question using the expression linking angular momentum and the conservation of energy, namely:

    E = U(r) + J^2/(2mr^2) + 1/2*m*r(dot)^2

    However, i cannot show the result that they want me to derive. There's a factor of 2 extra that I keep ending up with and I don't see how they have 8 in the denominator of one of the terms.

    Any help would be appreciated.
  2. jcsd
  3. Feb 13, 2010 #2
    Just bumping this -- any help at all would be great.
  4. Feb 13, 2010 #3
    Never mind -- I think I have the answer now.
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