An intuitive sense I get from writing is that prose possesses both inertia and momentum. When a project is conceived, even with a well defined and through outline I have hesitancy in actually starting the piece. Applying butt to chair and fingers to keyboard is always work it seems that the barriers at the start are high and steeper than later in the creation process.
Mind you this applies not just to the me getting started but also to the speed of the writing itself. Particularly with large works like novels at the start the words and scene do not come easily. It is as if the project is large, heavy, and I am trying to push it up a hill. As I get deeper into a project the words come faster and inertia seems to lessen, though it never goes away entirely.
The idea that writing has momentum is related but slightly different. In physics momentum has a vector, that is momentum describe both how much (usually velocity) something is moving and in which direction. Something with a large amount of momentum can be light but moving very fast or very large and moving slowly. Momentum measures how ‘resistant’ a moving object is to changing that vector.
When I near the start of a story for which I have an outline I will refer to that outline often the outline gives me the direction that the story needs to travel. But as I get deeper and deeper into the story I check the outline less and less. At this point the story is moving in its direction, the vector has been set and it fights a change in that. I find it fascinating that the shape of the story tends to confirm with what I laid out in the outline but now instead of checking that document for the next scene that outlines scene simply falls into place organically.
In the end I land where I had predicted, the story wraps up the way I expected and then I have start the process all over again for the next tale.