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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.
    ____kg*m^2
    (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.
    ____m*N

    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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