Blender Animation Expert for small task

I need to solve a 3D animation job that i have.

The project is simple: there are two conveyer belt (see photo) each of them has rigid body so that when an object is physically transported by the belt

each conveyer belt is animated to a constant velocity (let's say 1 m/s)

but i want that when a object is starting to roll on the convoyer A, convoyer B is stopping (gradually) and re-start spinning again after 2-3 seconds

NOTE: this dont have to be aimated by hand but its controller by the arrival of an object on conveyer belt A