Step 1 - FBD: Draw individual FBDs of the bar and the block.
Step 2 - Kinetics: Using your FBDs above, write down the Newton/Euler equations for the block and the bar. Be careful with your moment equation for the bar - since the bar does not have a fixed point, you need to use the center of mass of the bar for your moment equation.
Step 3 - Kinematics: You need to relate the acceleration of the block to the acceleration of the center of mass, G, of the bar through the rigid body acceleration equation:
aG = aO + αbar x rG/O - ωbar2 rG/O
Step 4 - Solve