# Question about circular motion and energy conservation

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.

• Yes, I would also be confused if a ball on a string was moving in a vertical circle and traversed the circle at constant speed. But that's not how the ball will move. (If this doesn't clear things up, could you please clarify your question.) Feb 24, 2021 at 23:02
• @tom10 Oh, I was under the impression that it would be possible for the ball to move uniformly. What would be another setup where it would not be possible for an object to move at constant speed? Feb 25, 2021 at 3:33
• Gravity creates an acceleration and acceleration is the definition of non-constant speed. So other situations where it's not possible for an object to move at constant speed are any other situations where the object is also experiencing an acceleration. Some other ways an object might be undergoing an acceleration are if it's being mechanically accelerated (like in an accelerating car or rocket or attached to a spring), or if it's charged and in an electric field, etc. Feb 25, 2021 at 3:41