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Homework Help: Why is this wrong?

  1. Sep 11, 2008 #1

    maniacp08

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    A horizontal force of 73 N pushes a 7 kg block up a frictionless incline that makes an angle of 20° with the horizontal.
    (a) What is the normal force that the incline exerts on the block?
    64.5N
    from cos(20)*68.67

    (b) What is the acceleration of the block? (Hint: assume up the block as the positive direction.)
    3.35m/s2

    from sin(20)*68.67 = 23.48
    F = MA
    23.48/7 = 3.35
     
    maniacp08, Sep 11, 2008
  2. jcsd
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  3. Sep 11, 2008 #2

    Doc Al

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    Staff: Mentor

    What about that horizontal 73 N force? Seems like you forgot all about it.
     
    Doc Al, Sep 11, 2008
  4. Sep 11, 2008 #3

    maniacp08

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    The 73N would affect the acceleration of the block right? I dont see how it affects the normal force.

    So should it be 73N - 23.48N = 48.52N.
    And plug that in the F = MA?
     
    maniacp08, Sep 11, 2008
  5. Sep 11, 2008 #4

    LowlyPion

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    Homework Helper

    That 73N force acts at an angle to the block. not parallel to the incline so there will be force transferred normally to the slope.
     
    LowlyPion, Sep 11, 2008
  6. Sep 11, 2008 #5

    Doc Al

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    Staff: Mentor

    Since the 73N force is horizontal (and the incline is not) it will affect both the acceleration and the normal force.

    Find the components of the horizontal force parallel and perpendicular to the incline.
     
    Doc Al, Sep 11, 2008
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