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Homework Help: How to find a force at a different angle?

  1. Sep 10, 2012 #1
    1. The problem statement, all variables and given/known data
    By finding angles, determine the component of the force acting at C which is perpendicular to the line CP. Show that the resulting moment agrees with the calculation in part (ii), if you were to look at that force alone.

    Force at C = 200Ni + 250Nj (calculated to be at an angle 75.96 angle from x axis)
    Force C magnitude = 320.156N

    2. Relevant equations

    Simple cos for x component and sin for y component, etc.

    3. The attempt at a solution

    This is part c of a problem. I have calculated that the line perpendicular to CP is 87.138 degrees off the x axis. So how do I find the component acting at that angle?

    I feel like this is really easy and I'm just overthinking things.

    I don't want the answer to the problem which is why I left out where the origin for the moment will be. I just want to know how to find this force.
  2. jcsd
  3. Sep 10, 2012 #2


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    Homework Helper

    Have you drawn a reasonably accurate diagram?
  4. Sep 10, 2012 #3
  5. Sep 10, 2012 #4


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    OK, that looks like the supplied diagram.

    Have you added in any lines of your own - like the line CP, then the components of the force parallel and perpendicular to that line?

    Once they are drawn, you should be able to calculate what you are after.
  6. Sep 10, 2012 #5
    I have it drawn out but I'm stuck though it seems obvious...

    I mean, it's just those 2 vectors with that theta in between. But you can't find the other magnitude with just one angle and one vector...

    Or do the two connect at their tips to form a right angle of some sort?
  7. Sep 10, 2012 #6


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    Your statement in red, above, is true for finding the x- and y-components. You will be using the the etc part of the statement.

    The Force vector forms some angle with the direction you seek - lets call it θ.

    The value of the component will be either Fc.cosθ or Fc.sinθ

    One of those values will be much bigger that the other.

    If you have a carefully drawn diagram, you will be able to see if it is the perpendicular or parallel component that is bigger - thus you know which component is which.

    Of course, if your diagram is so approximate that you draw the angles bearing little resemblance to their real values then you will be in great doubt about which one is which.
  8. Sep 10, 2012 #7
    Ahh I see now. I have a bad habit of not drawing diagrams or not doing them well enough to scale. Thanks for the help.
  9. Sep 10, 2012 #8
    And I got the wrong answer. My moments from that point using the two methods are not equal.

    I found the moment using x and y components timers perpendicular distance is equal to 52000.

    Using the second method, I found it to be 63879.

    I feel like I did the first method fine.

    For the second method I found the perpendicular angle to be 87.138 with a force magnitude of 319.7569N. The distance between point P and point C I found to be 200.2498m.

    Are those numbers correct>
  10. Sep 10, 2012 #9
  11. Sep 11, 2012 #10
    Supplying the whole question rather than the end parts makes us see the whole picture.
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