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Homework Help: What is the initial torque

  1. Oct 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A rod is initially along the y axis and has
    length ℓ. A force F =ˆı F1 +ˆj F2 + ˆk F3
    acts at the upper end of the rod.
    What is the initial torque ~τ about the lower
    end of the rod?
    1. Zero
    2. ˆıF3 ℓ
    3. −(ˆı F3 +ˆj F2 + ˆk F1) ℓ
    4. (ˆıF3 − ˆk F1) ℓ
    5. (ˆıF1 +ˆj F2 +ˆk F3) ℓ


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 18, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: Torque

    Hint: What's the definition of torque in terms of a cross product?
     
  4. Oct 18, 2009 #3
    Re: Torque

    I am not sure. Please help
     
  5. Oct 18, 2009 #4

    Doc Al

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    Staff: Mentor

    Re: Torque

    [tex]\vec{\tau} = \vec{r} \times \vec{F}[/tex]
     
  6. Oct 18, 2009 #5
    Re: Torque

    What answer would that be with respect to my choices?
     
  7. Oct 18, 2009 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Re: Torque

    That's for you to figure out! All I did was give a definition of torque. Hint: Find an expression for [itex]\vec{r}[/itex] (its magnitude is ℓ) and perform the cross product.
     
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