And to answer the original question, which I think I've figured out with a lot of thinking, is that force (of a physical object) doesn't exist until it interacts with something.
A bowling ball rolling down the lane doesn't have force until it hits the pins--then the force of the bowling...
Aha, I think the light bulb just went off with the terrm "reactive." Essentially what you're saying is that a given fixed object (ground, brick wall, etc.) simply applies -a to whatever force impacts it up to the breaking point, right?
Never quite thought of it as a passive or reactive...
But then we're back to the original issue: if F=ma and there's no a, then how do you have a non-zero F?
Boy, my head is starting to hurt from what should be a simple concept. Sorry if I'm being dense.
Okay, g-g=0 makes sense to me. BUT, if it's sitting on the ground, it's already at g-g, so my tugging at it does what? The net of the ground and my efforts are still g-g, so if I'm taking away some of a from the ground, then the ground is providing variable acceleration?
Or let's take me...
I'm still confused. If I try to lift a 1000kg weight, it's not going anywhere. I've done no work, and from my limited understanding I have not created/applied any acceleration, but I am still applying force to it, no?
Sorry, I'm feeling dense...
So, F=Ma. Just because F1-F2 = 0 (net force, zero acceleration) how do you have any "F"s to add or subtract, because you don't have any acceleration?
My couch must have force to balance out the force of gravity, but what is the acceleration of my couch? And...
Hopefully with my limited knowledge of physics I can make this make sense, and please correct me if I'm mis-using any terms.
What I am trying to figure out at least a rudimentary model or explanation for is how "work" peformed by the human body varies based on efficiency for the same "work"...