The sliding collar moves up and down the shaft, causing an oscillation of crank $\backslash ($ O B $\backslash ).$ If the velocity of $\backslash ($ A $\backslash )$ is not changing as it passes the null position where $\backslash ($ A B $\backslash )$ is horizontal and $\backslash ($ O B $\backslash )$ is vertical, determine the angular acceleration of $\backslash ($ O B $\backslash )$ in that position.