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Math Help - Mechanics / physics

  1. #1
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    Mechanics / physics

    The diagram shows four coplanar forces of magnitude SN, RN, 60N and 80N acting at a point 0 in the direction shown.



    Given the forces are in equilibrium, find the values of R and S.


    I try to solve S

    SNCOS60 = 60N
    SN = 60/cos60
    SN =120N

    Try to solve R

    RN + 120sin60 = 80N
    RN = 80 - 120sin60
    RN = -23.92


    Can anybody spot a mistake? I'm sure you use SIN for forces up and down and COS for forces horizontally.
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  2. #2
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    looking at it again would i find RN using 120sin30 ?
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  3. #3
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    what diagram?
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  4. #4
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    Its this 1



    seem this site doesnt host pictures for long lol
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  5. #5
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    Quote Originally Posted by djmccabie View Post
    Its this 1



    seem this site doesnt host pictures for long lol
    S \cos 30^0 = 60 .... (1)

    R + S \sin 30^0 = 80 .... (2)

    Solve equations (1) and (2).
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  6. #6
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    Quote Originally Posted by djmccabie View Post
    Its this 1



    seem this site doesnt host pictures for long lol
    might be because you posted it on your other thread ...

    http://www.mathhelpforum.com/math-he...-question.html
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  7. #7
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    As all forces are acting at one point, and that the point is being held in equilibrium by these forces it is said that the opposite forces are of equal magnitude. Mr Fantastic has detailed this with the two equations for vertical and horzontal force.

    Since

    S = (60/cos30)

    Then

    2R = 160-S

    This can be simplified to be....

    R = 80 - (30/cos30)

    Kind Regards,

    Ross
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