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Finding Friction force impeding boxs motion

  1. Nov 1, 2009 #1
    A 21.0 kg box is released on a 36.0° incline and accelerates down the incline at 0.271 m/s2. Find the friction force impeding its motion.

    F=mg*cos(36)

    F=21(9.8)*cos(36)
    F=205.8*.809
    F=166.495

    I'm not sure what I'm doing wrong. Please help
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 1, 2009 #2

    rock.freak667

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    Homework Helper

    the component of the weight acts downwards and the friction acts upwards (directions are parallel to the plane)


    so if the box moves down then ma=forces down-forces up.


    a bit easier now?
     
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