Dynamics: Angular Acceleration Problem

[vela]

$a_\text{Bx}=\omega_{B}^2R_\text{B/A} = -R_\text{B/A}\omega_B^2 = -R_\text{B/A}\omega_\text{B/A}^2 = -R_\text{B/A}(0)^2 =0?$

Think about this: Why are you singling out the x-component here? Couldn't you make the same argument for the y-component?