If we consider a ball attached to a string that moves in a circle at a constant speed, then in a ball-Earth system: the kinetic energy doesn't change, the string does zero work, and yet the gravitational potential energy is not the same throughout (for example, it's double at the top what it is at the bottom).
How can this be, if the system is isolated? I get that the ball doesn't have to move at a constant speed; I'm just confused about when it does.