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Homework Help: Find torque of a ball on the end of a light rod

  1. Oct 27, 2007 #1
    1. The problem statement, all variables and given/known data

    A small 1.25 kg ball on the end of a light rod is rotated in a horizontal circle of radius 0.890 m.
    (a) Calculate the moment of inertia of the system about the axis of rotation.
    (b) Calculate the torque needed to keep the ball rotating at constant angular velocity if air resistance exerts a force of 0.0800 N on the ball.

    2. Relevant equations

    a) kg*m^2
    b) kg*m^2-air resistance (dont know the formula; just a guess)

    3. The attempt at a solution

    a) 1.25*.89^2=.99kg*m^2 (i was able to figure out this answer)
    b) 1.25*.89^2-.08=.91m*N
  2. jcsd
  3. Oct 27, 2007 #2
    Hint: air resistance is a force, you are required to find torque. how do u corelate them?
  4. Oct 27, 2007 #3
    Do you know how to find find the Moment of inertia?
  5. Oct 28, 2007 #4
    You got the moment of inertia right.
    Carefully consider what forces it faces when moving at constant angular velocity.
  6. Oct 28, 2007 #5
    Find the torque which is applied by the air on the object.The same amount of torque should be applied in the opposite direction in the opposite direction.

    Torque = force * radius
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