Fill in the blanks:
$(a)$ If $\overrightarrow A \cdot \overrightarrow B = AB$,then the angle between $\overrightarrow A$ and $\overrightarrow B$ is ............
$(b)$ The velocity of a projectile at its maximum height is ............ (Take the angle of projection as $\theta$).
$(c)$ The projection of $\widehat i - 2\widehat j + 4\widehat k$ on the $y$-axis is ............

(A) Since $\overrightarrow A \cdot \overrightarrow B = |A||B| \cos \theta = AB \cos \theta$,given $AB \cos \theta = AB$,we have $\cos \theta = 1$,so $\theta = 0^\circ$.
$(b)$ At maximum height,the vertical component of velocity becomes zero,so the velocity is only the horizontal component,which is $u \cos \theta$.
$(c)$ The projection of a vector $\vec{V} = x\widehat i + y\widehat j + z\widehat k$ on the $y$-axis is the $y$-component,which is $-2$.