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Homework Help: Rotational motion - i just need help with net forces

  1. Oct 26, 2009 #1
    1. Mass m_1 on the frictionless table of the figure is connected by a string through a hole in the table to a hanging mass m_2.

    With what speed must m_1 rotate in a circle of radius r if m_2 is to remain hanging at rest?

    http://session.masteringphysics.com/problemAsset/1073602/3/knight_Figure_08_30.jpgp [Broken]

    2. F_radial = mv^2/r
    omega = v/r

    3. I know that F_z = 0 = normal - mg
    What is F_radial? I don't see how to connect the tension caused by m2 to force in the radial direction....
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 26, 2009 #2


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    If T is the tension in the string, acceleration of m_2 is given by
    mg - T = m_2*a.
    Similarly write down the expression for m_1. When the system is at rest a = ?
  4. Oct 26, 2009 #3
    I would have thought m_2*a = t - mg downward. This would not be motion in the radial direction. there is an inward radial force on m1 involving T. it's also obviously connected to m2, but how?

    m_1*a = -m_2*T is my intuition but I think it is wrong...........
  5. Oct 26, 2009 #4


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    The acceleration of both must be zero.
    The centripetal force is provided by the tension in the string.
    So T = ?
    The system will remain at rest it T - mg = ?
    Find the values of T from two equations and equate them to get the condition.
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