1. ## A simple question about vectors

Hello,
(see picture)
could you explain which is correct way to transform first expression into vectors (a or b) and why?

Thank you

2. ## Re: A simple question about vectors

If $R$ is the vector from $q_1$ to $q_2$ whose length is 1, then the correct expression is (b). This is because $a$ is the angle between $R$ and $u$, and $b$ is the angle between $R$ (plotted from $q_2$) and $v$, so $(\vec{R}\cdot\vec{u})=Ru\cos a =u\cos a$ and $(\vec{R}\cdot\vec{v})=Rv\cos b =v\cos b$. Dot product uses the angle between the two vectors. If the length of $R$ is not 1, then one has to divide by $R^2$ (in addition to the division by $c^2$).

3. ## Re: A simple question about vectors

None of the expressions are vectors???