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Math Help - cross product question

  1. #1
    ave
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    cross product question

    once I have determined the cross product of an angle, how do I tell if its facing up or down- ie a negative or positive angle- especially if im dealing with a plane that could be lying in any direction in 3d space
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    Quote Originally Posted by ave View Post
    once I have determined the cross product of an angle, how do I tell if its facing up or down- ie a negative or positive angle- especially if im dealing with a plane that could be lying in any direction in 3d space
    The cross product of an angle? Do you mean the cross product of two vectors lying at a given angle? And what, exactly, do you mean by "a negative or positive angle"? The direction of a line or vector in 3 space requires 3 angles. Since you specify "up or down", I would say just look at the z-component. If it is positive, up, negative, down.
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  3. #3
    ave
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    Quote Originally Posted by HallsofIvy View Post
    The cross product of an angle? Do you mean the cross product of two vectors lying at a given angle? And what, exactly, do you mean by "a negative or positive angle"? The direction of a line or vector in 3 space requires 3 angles. Since you specify "up or down", I would say just look at the z-component. If it is positive, up, negative, down.
    unfortunately because the vectors lying at a given angle as you correctly put it, could be anywhere in 3d space, the cross product doesnt necessarily lie on the z axis, that is exactly the problem....

    what I mean by a negative or positive angle is- lets assume the first vector is x=1, y=0, and the other vector is lying at y=1, x=0, that would be +90, where as if the second vector is at y=-1, x=0 , that would be -90 degrees

    now using the right hand rule I can tell which direction the cross product is going to face, unfortunately I am writing a computer program that plots points, so the right hand rule doesnt work for me... I want to know how to do this mathematically
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  4. #4
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    Quote Originally Posted by ave View Post
    unfortunately because the vectors lying at a given angle as you correctly put it, could be anywhere in 3d space, the cross product doesnt necessarily lie on the z axis, that is exactly the problem....
    I didn't assume the cross product was lying in the z-axis. I said "look at the z component of the cross product.

    what I mean by a negative or positive angle is- lets assume the first vector is x=1, y=0, and the other vector is lying at y=1, x=0, that would be +90, where as if the second vector is at y=-1, x=0 , that would be -90 degrees

    now using the right hand rule I can tell which direction the cross product is going to face, unfortunately I am writing a computer program that plots points, so the right hand rule doesnt work for me... I want to know how to do this mathematically
    The cross product of x_1\vec{i}+ y_1\vec{j}+ z_1\vec{k} and x_2\vec{i}+ y_2\vec{j}+ z_2\vec{k} is (y_2z_3- z_2y_3)\vec{i}+ (x_2z_1- z_2x_1)\vec{j}+ (x_1y_2- y_1x_2)\vec{k}. It is pointing "up" if x_1y_2- y_1x_2 is positive and "down" if negative.
    Last edited by mr fantastic; June 28th 2009 at 09:00 PM. Reason: Fixed close quote tag
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  5. #5
    ave
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    Quote Originally Posted by HallsofIvy View Post
    I didn't assume the cross product was lying in the z-axis. I said "look at the z component of the cross product.


    The cross product of x_1\vec{i}+ y_1\vec{j}+ z_1\vec{k} and x_2\vec{i}+ y_2\vec{j}+ z_2\vec{k} is (y_2z_3- z_2y_3)\vec{i}+ (x_2z_1- z_2x_1)\vec{j}+ (x_1y_2- y_1x_2)\vec{k}. It is pointing "up" if x_1y_2- y_1x_2 is positive and "down" if negative.
    oh ok I get it now- thanks for the help!! I really appreciate it
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