# Having problem in crossproduct, dotproduct.

1. Oct 2, 2007

### danong

[SOLVED] Having problem in crossproduct, dotproduct.

Sorry, i have a lil problem in solving for some math in physics equation.
I'm doing simulation of rigid body collision reaction,
and i applied the angular momentum,
which is L = p x r = I ( w2 - w1);
where p = mv is the impact of the collision,
w1,w2 is the angular velocity.
r be the vector from the center of body to the collision point.

I'm wondering if the eq is p x r,
in a 2D system, the third coordinate system is ignored.
and in some circumstances,
i let the equation to be reformed into something like p * n x r,
where n is the reflect/normal vector at the collision point.
since p x r is actually the cross-product,
but some books are telling me to take dotproduct of it,

i'm wondering and hoping to seek for help.

2. Oct 2, 2007

### Loren Booda

L=p x r= pr sin (a), where a is the smaller angle between p and r. The resultant vector L requires and resides in a third dimension by definition. The angular velocity omega (w), parallel to L, also lies in this third dimension.

Given p (momentum) and r (momentum arm), you do not find the angular momentum by taking their "dot product."

What two-dimensional system did you have in mind?

3. Oct 2, 2007

### danong

bleessh i just found that i got it figured, just it skipped few steps n i did proved it, not actually dotproduct of them, but cross of them ^^