The dot product of two vectors $\overline{P}$ and $\overline{Q}$ is zero if

$\overline{P}$ is a null vector and $\overline{Q}$ is a proper vector

$\overline{P}$ is a proper vector and $\overline{Q}$ is a null vector

$\overline{P}$ and $\overline{Q}$ are both null vectors 

$\overline{P}$ and $\overline{Q}$  are proper vectors perpendicular to each other