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Torque: Why does tension = Friction

  1. Feb 16, 2016 #1
    1. The problem statement, all variables and given/known data
    A 10.0 kg sphere is attached to the incline by a horizontal string. Whats the tension in the string.
    media%2F2cb%2F2cb72ecc-ce74-4a5d-acae-7e9c082d68a4%2FphpU05dAk.png

    2. Relevant equations
    sum of all horizontal forces = 0
    sum of all vertical forces = 0
    sum of all torques = 0

    3. The attempt at a solution
    The sum of the horizontal forces
    is -Tsin55 degrees - F + mgsin35 degrees = 0
    Why does F= T????
    I know F= T because when I substitute F=T and solve for T I get the right answer (30 .9 N) but why is this?
     
  2. jcsd
  3. Feb 16, 2016 #2

    haruspex

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    If you want a quick way to get a relationship between F and T, the trick is to take moments about a point where only those two forces have a moment. What other forces are present? What point do none of those have a moment about?
     
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