in the reference frame of the sphere, there are two forces: g(downward gravity) and horizontal inertial force a,
let the particle slide an angle phi, Work done by the two forces=mRasinθ +mRg(1-cosθ)=mv2/2,
v2=2R(asin(phi)+g(1-cos(phi)), v=√(2R(asinø+g(1-cosø)))