I don't understand how does a rotating moment caused by the weight force form when a submerged object is tilted like this. I understand that the buoyancy force produce a moment about CG because of the horizontal distance between FB and CG. But for the weight how can it rotate the object about its center of gravity when the Force is directed through the Center of gravity . How would it create a torque? A free body always create a torque about its CG.
I understand that the buoyancy force produce a moment about CG because of the horizontal distance between FB and CG.
Actually, the upward buoyant force and downward weight, which are parallel equal but opposite forces, form what is called a "couple". A couple causes pure rotation without translation (since the net force is zero). The magnitude of the couple is the sum of the torque contributions of each of the two forces, or
$$M_{C}=F\frac{d}{2}+F\frac{d}{2}=Fd$$
where $d$ is the perpendicular distance between the lines of action of the two forces, and $F_{B}=W=F$ for a neutrally buoyant body.
When the ensuing rotation caused by the couple vertically aligns the two forces (after some possible oscillations), the body will be rotational equilibrium.
Hope this helps.
A freely falling mass without other forces on it does not experience a torque.
A torque is the result of two equal anti parallel forces displaced sideways. See Toppling of a cylinder on a block.
You can get a torque when a ball rolls down an inclined plane if there is friction. Is work done by torque due to friction in pure rolling?
In your example, torque is the result of buoyancy and weight acting together.
