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Math Help - Vector direction

  1. #1
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    Vector direction

    Hi,
    Its a long time since i did any maths so i'm struggling with this and would really appreciate some help. Basically i'm using a computer software that calculates the force acting on an object. Normally i get the x y and z vector components of the force i.e (1,0,0) etc. Unfortunately, now i need to work out the force in a certain direction. I know the angles this direction makes with the x, y and z components (7.7,19.9 and 68.5 degrees respectively). So i need to find out what the vector in 3D space is that makes this angle i.e. the x,y and z components of the new vector. I've tried using the dot product so that i get, for example
    cos 7.7=(A.B)/(|A||B|)
    So i know A (1,0,0) and I think i can assume that the length of B(x,y,z) is also 1. So I'm left to solve for x for vector B when i calculate the dot product. If I repeat to get the dot products of B with C(0,1,0) and B with D(0,0,1), I also get values for y and z, resulting in B=(0.99,0.94,0.36). I hope I've explained this ok, but am I correct in with i'm trying to do (is this the vector in the direction i'm looking for)? Any thoughts really appreciated!

    Thanks
    David
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  2. #2
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    Quote Originally Posted by molonski View Post
    Hi,
    Its a long time since i did any maths so i'm struggling with this and would really appreciate some help. Basically i'm using a computer software that calculates the force acting on an object. Normally i get the x y and z vector components of the force i.e (1,0,0) etc. Unfortunately, now i need to work out the force in a certain direction. I know the angles this direction makes with the x, y and z components (7.7,19.9 and 68.5 degrees respectively). So i need to find out what the vector in 3D space is that makes this angle i.e. the x,y and z components of the new vector. I've tried using the dot product so that i get, for example
    cos 7.7=(A.B)/(|A||B|)
    So i know A (1,0,0) and I think i can assume that the length of B(x,y,z) is also 1. So I'm left to solve for x for vector B when i calculate the dot product. If I repeat to get the dot products of B with C(0,1,0) and B with D(0,0,1), I also get values for y and z, resulting in B=(0.99,0.94,0.36). I hope I've explained this ok, but am I correct in with i'm trying to do (is this the vector in the direction i'm looking for)? Any thoughts really appreciated!

    Thanks
    David
    Using A= (1, 0, 0) you get cos(7.7)= B[sub]x[sub], the x component. Taking A= (0, 1, 0) you get cos(19.9)= By and taking A= (0, 0, 1), you get cos(68.5)= Bz. Looks like what you have done (though I would round cos(68.5)= 0.3665 to 0.37 rather than .36).
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  3. #3
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    Thanks for the reply HallsofIvy.
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