Cross Product Confusion

1. Nov 17, 2011

SeannyBoi71

Hi all,

Just having some trouble understanding a certain example of cross product. It's actually for physics, but figured it belongs in this forum.

In the question I am supposed to cross (dl)y hat with (x1)x hat.

So I go (0 dl 0) x (x1 0 0) = (-x1dl)z hat. But turns out the answer the computer looking for is the same but without the negative. Wouldn't that be the case if i crossed it backwards, i.e. (x1)x hat cross (dl) y hat ??

2. Nov 17, 2011

sandy.bridge

Yes,
$$(0, dl, 0)\times{(}x_1, 0, 0)=-xdlk$$
and
$$(x_1, 0, 0)\times{(}0, dl, 0)=xdlk$$

3. Nov 17, 2011

SeannyBoi71

Well then if that's the case I'm still confused. Because then my answer should be right.

4. Nov 17, 2011

mathman

You need to write the question exactly as it presented, since the cross product depends on which comes first.

5. Nov 17, 2011

SeannyBoi71

Ok, I am thinking that there is an error in the system, because according to the Biot Savart Law it has a dl x r in the numerator, where r = (x1 0 0) and dl = (0 dl 0). So I computed that but yet my answer was wrong.

6. Nov 18, 2011

mathman

Check the computer program specs. Presumably it will tell you how it interprets a cross product for the order in which the vectors are input.