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Homework Help: Rod - angular acceleration

  1. Feb 24, 2005 #1
    rod -- angular acceleration

    A long uniform rod length L and mass M is pivoted about a horizonal, frictionless pin passing through one end. The rod is released from rest in a vertical position. The instant the rod is horizontal, what is the magnitue of its angular acceleration.

    It's angular speed I know is [tex]\sqrt{\frac{3g}{L}}[/tex]
    using the formula [tex]\alpha=rw^2[/tex]
    I get then angular acceleration to be 3g. However the book tells me that the answer is 3g/2L. What am I missing?
  2. jcsd
  3. Feb 24, 2005 #2


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    Whatever formulas are you using???

    Write down the moment-of-momentum equation about the pivot:
    a) What is the moment of inertia?
    b) What is the torque induced by gravity?
  4. Feb 24, 2005 #3


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    Yes,your answer is incorrect,even not knowing the physics to solve the problem,nor the book's answer.

    I think you can use the law of conservation of energy...

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