Wondering how they got BB8 to move?
When the center of mass shifts away from being vertically over the contact point, there is an external torque on the sphere. This torque then increases the angular momentum of the sphere and causes it to roll. Once the sphere starts rolling, the inside mass could just stay at the lowest point—except there is some external friction which will require the inside mass to continue to ride up the side and provide some torque.
To read the rest of the article please go to:Wired