A force $\vec F=2\hat i+3\hat j$ is acting on a point $(1,0,3)$. Find the torque of this force about the axis along $y=3x$.
I know 3-D geometry. So, I can handle equations of lines in 3-D too.
Definition of torque: $$\tau(\text{torque})= \vec r \times\vec F $$
where $\vec r$ is the position vector of point of application of force about the axis.
I can't find $\vec r$ correctly. Can someone help me out?
To find out $\vec r$ we need two point. From $y=3x$ we can say that (0,0,0) is situated on it and $\vec F$ is working on $(1,0,3)$ so . $$ \vec r=(1-0) i+(0-0)j+(3-0)K $$ $$\vec r=i+3k $$