Hi !
If we consider any kind of engine (machine that uses energy to develop mechanical power ),let's make a simple analogy:
-"inbox" engine (let's say,an old internal-combustion engine ),with an open view of his spare parts:
-hand crank (to start motion)
-the properly engine (to impart motion)
-flywheel (to store motion)
-"outside" engine (let's say,a possible use of gravity power ),with an open view of his spare parts:
- "eccentric" mass (to start motion)
-gravity,as a "natural engine" (to impart motion) is...or,you can take it as a "ready-made" (but unseen!) spare part
-flywheel (to store motion)
So,as a sum ,remains two spare parts,or a simple attachment to this huge ,but invisible natural engine/gravity.
All the best! / Alex