The block $A$ and attached rod have a combined mass of $60 \mathrm{kg}$ and are confined to move along the $60^{\circ}$ guide under the action of the 800 -N applied force. The uniform horizontal rod has a mass of $20 \mathrm{kg}$ and is welded to the block at $B$. Friction in the guide is negligible. Compute the bending moment $M$ exerted by the weld on the rod at $B$