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Homework Help: Rotational Motion change angular rotational

  1. Oct 31, 2011 #1
    1. The problem statement, all variables and given/known data

    9. A force can change angular rotational if it acts through:

    a. the axis of rotation

    *b. a lever arm tangential to the direction of rotation

    c. a moment arm parallel to the axis of rotation

    d. none of these

    2. Relevant equations

    n/a

    3. The attempt at a solution

    I would like to understand why b is the correct answer.
     
  2. jcsd
  3. Oct 31, 2011 #2
    I guess we could start with why it isn't a or c. Angular momentum is changed when Torque is applied. If you apply a force through the axis of rotation, then r is 0. if you apply force through a moment arm parallel to the axis of rotation then torque is also zero since

    τ = F x r
    |τ| = Frsinσ

    since F and r are parallel, the angle is 0, therefore the torque is 0. If you applied it tangentially, then the σ value will be 90 and sinσ will be 1.
     
  4. Oct 31, 2011 #3
    Got it...thanks!!
     
  5. Oct 31, 2011 #4
    Awesome. youre welcome :smile:
     
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