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Homework Help: Calculating tension in rotational kinematics

  1. Sep 25, 2011 #1
    1. The problem statement, all variables and given/known data
    A block with mass m is revolving with linear speed v1 in a circle of radius r1 on a frictionless horizontal surface (see the figure ). The string is slowly pulled from below until the radius of the circle in which the block is revolving is reduced to r2.

    Calculate the tension T in the string as a function of r, the distance of the block from the hole. Your answer will be in terms of the initial velocity v1 and the radius r1.

    2. Relevant equations
    I really have no idea. I'm going to assume it involves conservation of angular momentum: L = r X mv

    3. The attempt at a solution
    L1 = r1 * m * v1
    L2 = r * m * v2

    L1 = L2
    r1 * m * v1 = r * m *v2
    r1 * v1 = r * v2

    Now i'm stuck

    Thank you for the help
  2. jcsd
  3. Sep 25, 2011 #2
    Nevermind, using T = (mv^2) / r, and setting L1 = L2, I found v to be (v1*r1) / r

    Plugged that into the tension and found T = (m*v1^2*r1^2)/r^3
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