energy. Gravity does not expend energy, therefore gravity is not a force.
Doubtful bit of logic there but I think I see what you mean. All the energy originally came from your muscles, and it was just held by the gravitational field for a while, then released to some other form when the rock fell off.
energy put into the rock you are lifting, why does gravitational attraction not increase with distance, as would be the case if you were stretching a spring? the higher you lift the rock the more potential energy you impart to it.
The higher you lift it, also the further it can drop, so the more energy is released. I suppose that's where the saying "the higher they climb the harder they fall" comes from :P For a spring the relationship between extension and force is different, and can be customized by designing the spring differently. But still, whatever energy you put in is available to be extracted again.
What, then, about a spacecraft that travelled
from Earth to the moon, why would it be attracted by the
Because the effect of the moon's gravity becomes greater than the earth's only when you're up close. It's a bit like having a magnet on the ceiling - throw a paperclip up and it usually falls down, but if it gets close enough to the magnet then it can be held up indefinitely. It still has potential energy ready to be released by falling down, but to release that energy you first have to input some energy.
Magnets, moons and springs are all just devices for storing energy not supplying it. You can't get anything out of them without putting something in.
Returning to the rock, if, having picked it up you altered your position so that you were holding your rock over an open well; when you released it you would be very surprised
Yes but the rock already had some extra potential energy before you picked it up, just by being on the ground above the bottom of the well. It's that energy that's released.
