Posted by **Wykeena** on Sunday, June 12, 2011 at 7:49pm.

The problem concern the position vectors r1=2.43mi-4.74mj and r2=-3.02mi+0.85mj.

Find the angle between the two vectors. Hint: Find the angle each makes with the +x-axis.

- physics -
**bobpursley**, Sunday, June 12, 2011 at 8:19pm
do the dot product, and the magnitude of the cross product.

cross product= r1XR2=magR1*magR2*sinTheta

dot product= magR1*magR2*cosTheta

cross product/dot product = tan theta

solve for theta.

How do you get cross product?

r1XR1=(2.3i-4.74j)X(3.02i+.85j)

= 2.31iX(3.02i+.85j) + (-4.74j)X(3.02i+.85j)

well iXi=0, iXj= k, jXi=-k, jXj=0

or = 2.31*.85k+4.74*3.02k=CCCCc

How do you get the dot product?

r1.r2= 2.3*3.02-4.74*.85=DDDDD

so tan theta = CCCCC/DDDDDD

- physics -
**bobpursley**, Sunday, June 12, 2011 at 8:21pm
I see I missed a sign on R2, correct that please.

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