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Homework Help: Resolving [using sine/cosine]

  1. Apr 19, 2010 #1
    1. The problem statement, all variables and given/known data

    A box is held at rest on a smooth slope. The slope is inclined at an angle of 18◦ to the
    horizontal. Given that the normal reaction force between the box and the slope is 79 N, what
    is the mass of the box?

    2. Relevant equations
    none


    3. The attempt at a solution

    well, i posted a relatively easy example but i'm finding it hard to resolve it. i know about this triangle

    aox0za.png

    but im not too sure when i'm meant to use it and how it works.

    diagram annotated replies would be a real help.
    thanks!
     
  2. jcsd
  3. Apr 19, 2010 #2

    tiny-tim

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    How is it held at rest?

    If the force holding it isn't parallel to the slope, it will alter the reaction force. :confused:
     
  4. Apr 21, 2010 #3
    well im not too sure :S but thats the question
     
  5. Apr 21, 2010 #4

    tiny-tim

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    ok, well let's assume that the force holding it is parallel to the slope. :smile:

    Your diagram only shows how to resolve a force F into components along two perpendicular axes … it doesn't really help with this question.

    You need a vector triangle or free body diagram showing the three forces on the box (weight, normal force, and holding force). :wink:
     
  6. Apr 22, 2010 #5
    Answer deleted.

    OP use Newtons law..
     
    Last edited: Apr 22, 2010
  7. Apr 22, 2010 #6

    tiny-tim

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    Susanne, please don't give the answer. :wink:
     
  8. Apr 22, 2010 #7
    There ;)
     
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