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    Figure 2 shows 4 points A, B, C and D arranged such that they form the corners of a square of
    side 2 m. Forces of 5 N, 3 N, 2 N and 4 N act in the directions AB , BC , DC and DA
    respectively.
    (a) Calculate the magnitude and sense of the resultant moment about A. (3 marks)
    An additional force of magnitude X Newtons is added in the direction CA . The resultant
    moment of all the forces about D is now zero.
    (b) Find, in the form k√2, the value of X.

    i got part a but on part b i keep getting the distance of X from d to be root 2 not 2root2 which is what it says on the mark scheme?
    any ideas?
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    (Original post by Lola1244)
    Figure 2 shows 4 points A, B, C and D arranged such that they form the corners of a square of
    side 2 m. Forces of 5 N, 3 N, 2 N and 4 N act in the directions AB , BC , DC and DA
    respectively.
    (a) Calculate the magnitude and sense of the resultant moment about A. (3 marks)
    An additional force of magnitude X Newtons is added in the direction CA . The resultant
    moment of all the forces about D is now zero.
    (b) Find, in the form k√2, the value of X.

    i got part a but on part b i keep getting the distance of X from d to be root 2 not 2root2 which is what it says on the mark scheme?
    any ideas?
    Could you show us a picture of your working or a diagram?
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    (Original post by Zacken)
    Could you show us a picture of your working or a diagram?
    i just dont get how they have calculated the perpendicular distance????

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    They've calculated the perpendicular distance using pythagoras theorem to give square root of eight which equals 2 root 2. Now there are two forces with a total of 16Nm acting anticlockwise of D, so there has to be 16Nm acting clockise. so
    2√2*F=16
    F=16/2√2
    F=8/√2
    F=4√2 N
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    (Original post by Dominator1)
    They've calculated the perpendicular distance using pythagoras theorem to give square root of eight which equals 2 root 2. Now there are two forces with a total of 16Nm acting anticlockwise of D, so there has to be 16Nm acting clockise. so
    2√2*F=16
    F=16/2√2
    F=8/√2
    F=4√2 N
    Can you explain how to do part a? Thanks.
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    (Original post by Lola1244)
    i just dont get how they have calculated the perpendicular distance????

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    Hey could you please explain with you did part a? Did you work out the diagonal distance? I don't get what you really need to work out... Thanks!
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    (Original post by coconut64)
    Hey could you please explain with you did part a? Did you work out the diagonal distance? I don't get what you really need to work out... Thanks!
    This was done by taking moments about A. But the key point is that forces that act through the point that you are taking moments about have a moment of 0Nm. Thus in the diagram the 4N force and the 5N force act through A so each have a moment of 0. However, the 2N and the 3N forces have moments. The 3N force acts anticlockwise with moment 3x2=6Nm and the 2N force acts clockwise and has moment 2x2=4Nm. Therefore the resultant moment is 6-4 = 2Nm
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    (Original post by Cryptokyo)
    This was done by taking moments about A. But the key point is that forces that act through the point that you are taking moments about have a moment of 0Nm. Thus in the diagram the 4N force and the 5N force act through A so each have a moment of 0. However, the 2N and the 3N forces have moments. The 3N force acts anticlockwise with moment 3x2=6Nm and the 2N force acts clockwise and has moment 2x2=4Nm. Therefore the resultant moment is 6-4 = 2Nm
    What distance are you using because I don't get how you can work out the distance ( 2m)? Where does it come from ? Thanks. ps I get why only two forces are involved .
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    (Original post by coconut64)
    What distance are you using because I don't get how you can work out the distance ( 2m)? Where does it come from ? Thanks. ps I get why only two forces are involved .
    The distance is given in the question.
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    (Original post by Dominator1)
    The distance is given in the question.
    OH so each side would just be 2m. But how do you know whether the force acts in the clockwise direction or anti clockwise direction? Thanks.
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    (Original post by coconut64)
    OH so each side would just be 2m. But how do you know whether the force acts in the clockwise direction or anti clockwise direction? Thanks.
    Imagine that you have this as a wooden square in front of you. Now put a nail through point A (only point A) so you cannot move that edge. Let four of your freinds apply the forces shown in the question by their fingertips in the directions shown. Once you visualise that you'll get it.
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    How did you get the perpendicular distance as 2√2? If you do sqrt(2^2+2^2)=2√2 then you have to half it because that's double the perpendicular distance. I think it would be sqrt 2. I will need to ask my teacher but it could be a mark scheme error.
 
 
 
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