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How to find forces acting on object on a slope?

  1. Oct 22, 2013 #1
    1. The problem statement, all variables and given/known data


    an object is at rest on a slope at angle of σ

    Using trigonometry, solve the forces acting at Ft and Fn

    2ezlnqa.jpg



    2. Relevant equations

    Soh
    Cah
    Toa



    3. The attempt at a solution

    sin σ = G/Ft
    sin σ /Ft = G
    G/ sin σ = Ft (my final answer)

    Book says correct answer is Ft = G sinσ


    Fn = ??? I wanst even sure how to strat this using σ

    The book says the correct answer is Fn = G cosσ
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Oct 22, 2013 #2

    NascentOxygen

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    Staff: Mentor

    The force G can be resolved into 2 components: one which is normal to the slope (the exact opposite to Fn), and one which is parallel to the slope (the exact opposite to Ft).

    Draw the triangle of forces showing how G equals the vector sum of these two forces.
     
  4. Oct 22, 2013 #3
    edited...I'll be back
     
    Last edited: Oct 22, 2013
  5. Oct 22, 2013 #4
    Thanks NascentOxygen. Unfortunately I dont quite get why the answers are what they are. I have drawn myself a new triangle which shows the two vector forces but I still don't understand where the answers have come from.

    Especially Fn = G cosσ. I thought cos in basic terms meant adjacent/hypothenuse . So if the sloped line is the hypothenuse and the bottom line is the adjacent once I draw the line which is exact opposite to Fn I now no longer have a right angle triangle.....arghh!! I'm just so confused...
     
  6. Oct 22, 2013 #5

    NascentOxygen

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    Staff: Mentor

    Show your diagrams.
     
  7. Oct 22, 2013 #6
    here is my latest diagram
    2yw8ygz.jpg

    I'm now starting to think Soh Cah Toa isn't actually relevant there ?

    Fn = G cosσ Ft = Gσ

    I'm unsure where these two formulas/equations have come from and why Soh Cah Toa isn't relevant here...

    I must be missing something fundamental
     
  8. Oct 23, 2013 #7

    NascentOxygen

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    Staff: Mentor

    It's the triangle beneath the object which you need to focus on.
    redraw it separately, and larger, and label what you know. Its vertical line represents G Newtons.
     
  9. Oct 23, 2013 #8

    NascentOxygen

    User Avatar

    Staff: Mentor

    It's the triangle beneath the object which you need to focus on. Redraw it separately, and larger, and label what you know. Its vertical line represents G Newtons.
     
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