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Finding the resultant of coplanar forces problem

  1. Dec 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A uniform ladder of weight W rests with its top against a rough wall and its foot on rough ground which slopes down from the base of the wall at 10 degrees to the horizontal. Resolve, horizontally and vertically, each of the forces acting on the ladder.

    2P2cHlT3Aq0c.jpg

    2. Relevant equations


    3. The attempt at a solution

    Okay so I'm just having some difficulty following this example about coplanar forces from my book.

    Looking at the smaller diagrams they drew for splitting R2 and F2 into their perpendicular components, I can't seem to figure out how they knew that those angles would be 10 degrees. I know I need to go back to the original diagram and employ some basic geometrical reasoning, starting from the fact that the ground is inclined at 10 degrees. However, I've been fiddling about with this for a while (drawing on the components on the main diagram, forming right triangles, looking for any parallel lines etc) to try and figure out how those angles would be 10 degrees myself but haven't made any headway. So I'm think I'm going about this the wrong way/not noticing something important.

    Any hints/tips to help me on this one would be much appreciated :)
     
  2. jcsd
  3. Dec 6, 2015 #2

    Mark44

    Staff: Mentor

    Turn the first of the two triangles they drew around. ##R_2## is normal to the ladder, so a vertical line starting from the bottom of the ladder makes an angle of 10° with ##R_2##. Once you see that, getting the two sides of the triangle are pretty straightforward.
     
  4. Dec 6, 2015 #3
    Thanks for the response - I think I can see now how the smaller diagrams fit in with the bigger one. The only thing is I don't get how they knew that the angle between R2 and the vertical line drawn is 10 degrees. How?
     
  5. Dec 6, 2015 #4

    Mark44

    Staff: Mentor

    The vertical line is perpendicular to the horizon, right? Through what angle would you have to rotate another vertical line to get one that is perpendicular to the sloping ground? Where you end up is ##R_2##, the hypotenuse of that right triangle.
     
  6. Dec 6, 2015 #5

    haruspex

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    What angle is F2 to the horizontal? What angle is R2 to F2? What angle does that make R2 to the horizontal?
     
  7. Dec 6, 2015 #6
    F2 is 10 degrees to the horizontal. R2 and F2 are perpendicular so 90 degrees. So does that mean that R2 to the horizontal is 100 degrees?
     
  8. Dec 6, 2015 #7

    haruspex

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    Yes. So what is the angle of R2 to the vertical?
     
  9. Dec 6, 2015 #8
    Ahh I think I see now. Given that R2 is 100 degrees from the horizontal, and drawing in a vertical line perpendicular to the horizontal at the foot of ladder, its clear that the angle R2 to the vertical will be given by 100-90 .So R2 to the vertical is 10 degrees and that's where the 10 degrees comes from in the component diagrams. Thank-you very much for the hints :)
     
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