# cross product

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bitshit    163
Hi, I've been using the crossproduct to calculate the normal vector for two given vectors in 3d space, which I calculate by doing: v × w = (vywz − wyvz, vzwx − wzvx, vxwy − wxvy ) I tried to see why this calculation results in a vector perpendicular to the plane v & w form, but can't figure out why that is. In 2D for 1 vector it is simple enough, you just switch (x,y) with (-y,x)to get a perpendicular vector. But I can't see why the crossproduct does this in 3d for two vectors... Could anyone explain me why? [Edited by - bitshit on October 13, 2007 1:48:29 PM]

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Wiggin    374
This is quite easy to show if you know that the dot product of two non-zero vectors is 0 if and only if they are perpendicular.

v * (v x w) = vxvywz - vxwyvz + vyvzwx − vywzvx + vzvxwy − vzwxvy = 0

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Zipster    2359
What you can do is plug the unknown output vector (x,y,z) into the top row of a 3x3 matrix, where the other two rows are the input vectors (vx,vy,vz) and (wx,wy,wz):
[  x  y  z ][ vx vy vz ][ wx wy wz ]

We know that the determinant of a matrix should be the hyper-volume of the parallelepiped formed by the vectors of that matrix. Since we want (x,y,z) to be perpendicular to the input vectors, the parallelepiped is going to look like a parallelogram that also has a height, with no sheering. Volume is base area times height. It just so happens that the length of the cross product is both the base area and the height, meaning that |(v x w)|2 = volume = determinant (the squared length of the cross product equals the determinant). If we expand the above determinant we get:

x(vywz - wyvz) + y(vzwx - vxwz) + z(vxwy - vywx) = determinant

Clearly, if we let x, y, and z equal their constant factors from the above equation, we satisfy all our conditions.

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johnb    351
Perhaps the easiest way is as follows.

if p = (1, 0, 0) and q = (0, 1, 0)

There are two candidates for the perpendicular to these two vectors, p ^ q, and we chose p ^ q = (0, 0, 1) not (0, 0, -1) by convention.

Note that this is the vx.wy of

(vy.wz − wy.vz, vz.wx − wz.vx, vx.wy − wx.vy)

Then if r = (0, 0, 1) you can similarly write down p ^ r, r ^ p. Reversed they are negated as the cross product is anti-symmetric. You should find they correspond to the other terms in the formula for v ^ w. The rest follows by linearity, i.e. because

(a ^ b) + (a ^ c) = a ^ (b + c)

and m(a ^ b) = (ma) ^ b

You can combine them to produce the full cross product.

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bitshit    163
Thanks guys! The penny dropped :)