Let $\overset{\to }{\mathop{A}}\,$ be a unit vector along the axis of rotation of a purely rotating body and $\overset{\to }{\mathop{B}}\,$ be a unit vector along the velocity of a particle P of the body away from the axis. The value of $\overset{\to }{\mathop{A}}\,.\overset{\to }{\mathop{B}}\,$ is