....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
.....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