# 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.