A book I have somewhere, but never got around to wading through. When one looks *very* closely, most of our concepts of 'energy', its 'conservation' etc. get pretty sticky, much as the Compton effect, matter / antimatter annihilation all seem to reduce the entire pile of stuff to,

Energy=Mass=Energy= which would you like today, sir?

So all I can say about 'force' is, some measure it in dynes. And to do the solar wind thing? don't see how gravitation (well, except for the local stuff around Sol) is more than a slingshot for KE and.. F=MA. (if gadget is accelerating (compared to Sol) - energy Was added!)

But as Feynmann's dad 'explained' to the kid about inertia, after the wonderful ball-in-a-wagon experiment - nobody knows 'why' !!

Me neither,

A.