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Homework Help: Torque problem

  1. Jan 15, 2010 #1
    1. The problem statement, all variables and given/known data

    cubical block of mass m ,edge a,slides down the inclined plane of inclination [tex]\varphi[/tex] with uniform velocity.
    torque of normal reaction on the block about its centre is?

    3. The attempt at a solution
    Torque=mgcos[tex]\varphi[/tex] X a/2
    mgcos[tex]\varphi[/tex]sin[tex]\varphi[/tex]a/2
    Book says:
    mgsin[tex]\varphi[/tex]a/2
     
  2. jcsd
  3. Jan 15, 2010 #2

    ideasrule

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    Homework Helper

    Think about it this way: gravity acts on the center of mass, so gravity contributes no torque. Friction does, and the normal force has to balance the torque that friction creates.
     
  4. Jan 15, 2010 #3
    So that should make the answer mgcos[tex]\varphi[/tex]sin[tex]\varphi[/tex]a/2
    and not
    mgsin[tex]\varphi[/tex]a/2
    Right?
     
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