"inside vs. outside"

[iacob alex]

    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