Really Thnx for your explication, but I can't understand why dA is equal to 2πRsinθRdθ and why are you using cilindrical coordenates instead of spherical.
I've tried to solv for dA = sinϕdϕdθ and what I've got was ρ4 π r²
Two hollow spheres, both the mass and radius R M , which are rotating around a center of mass ( CM ) , with an initial period To, are kept distant from each other by an ideal wire with a distance of 8R. At a given instant a motor is driven by wrapping the wire and making the two spheres meet...