An "elementary" point of view...

[iacob alex]

   ....related to a possible  PM,is to bring to light the "prime mover".
   
      If you take a short look at an overshot water mill,you can notice easy:

       -without the river flow,the energy collector (the "wheel"...),is not moving...is well balanced.A pair of scales,in the fulcrum "feels" no difference.

       -with the river flow,the wheel moves...a pair of scales,in the fulcrum, "feels" the difference.

       As an abstract,existing only in the mind,we can keep this word :the difference.

       It can be a concrete thing:the overshot river flow,acts as a prime mover,due to gravity.

       It can be any possible object(s) of thought("difference" ),also...if it can work as a prime mover,due to gravity too.

          All the best! / Alex

[iacob alex]

.....can be also,a power collector(usually a certain shape of wheel),immersed in a flow:equal arms,concave-convex shape.
   
    For gravity,the  shape  is not important...CoG ,only.

    The shape of mass is "transparent" for gravity,not for nuclei.

    It's possible to lay the nuclear system of mass,in such a manner,alike concave-convex shape of a common wheel?

    A kind of nanotechnology marvelous accomplishment? Who knows?

    For the moment,thinking in a classical way,we have the possibility  to imagine a long arm-short arm(equal masses case) ,vis-a-vis fulcrum.

    A formal image is a feedback energy system,with input<<<output,due to an "inner" mass circulation/flow.

     All the best! / Alex