Momentum

 Story moves forward when a character who wants something takes a step toward achievement.  That single step of acting on an agenda has triggering effects, setting off collateral waves of supportive movement and either direct opposition or the defensive responses of collateral characters who may be directly affected of have reason to fear they will feel some impact if they do not move out of the way or build up a sizable defense mechanism.

Story is like the principals of movement, friction, and gravity, the forces governing objects in motion and those forces intent on bringing movement to a halt.  Enter now that near delicious word, also borrowed from the study of objects in motion, which is to say momentum.

Spending time to familiarize yourself with the laws and conventions relative to motor vehicles on California roads and highways, preparatory to the written examination required every four years, you see a few things that are good candidates for things to be memorized.  You also see charts and demonstrations regarding the effects of collisions when a vehicle reaches a significant velocity.

From the beginning, story requires visual effects of velocity, of objects in motion.  You pay particular heed to opening velocity, how a story sets its individuals into meaningful action instead of choruses or prefaces or other indulgences into the worlds of backstory, those events that happened before the present story begins, which have significant effect on the characters, conflicts, and conventions describing the story under review.

Let's say a good-sized boulder comes crashing down a mountainside, providing a splendid example of opening velocity.  Pretty straightforward, no?  Into the equation, you being you, the rogue boulder--not a bad name for a story--reminds you of one of your pals, Sisyphus, doomed to an eternity of pushing a good-sized boulder up a steep hill, only to see it roll down again.  You being you, the story of this rogue boulder will in fact involve Sisyphus, who may have grown a bit drowsy, given the boring nature of his job, and consequently failing to keep his rock under prudent control.

This could lead to a cry from an outraged villager, hurling imprecations in Sisyphus' general direction, or an intervention team, come to make sure Sisyphus does not get his rock close to private homes, where it might cause damage as it barrels its way down the side of the hill.  In such a scenario, Sisyphus would have to fight the intervention team for the right to carry out his eternal sentence of pushing the rock up the side of a hill.

Nothing in his orders say anything about such niceties as due diligence, only that he push the rock up one side of the hill to the point where gravity carries it down the other side, whence it continues to roll according to its own mass, the downward grade of the hill, the gram-molar weight of the boulder, thus that remarkable borrowed word, momentum, which is the force of an object, which in itself is measurable if we know the size of the rock, the degree of the downward slope, the weight of the rock.

Imagine, if you will, Zeus, who had in fact set this eternal punishment of Sisyphus, coming to Sisyphus to complain.  "Now see here, Sis.  We've been getting complaints about you allowing that rock to get away from you, and if you don't shape up, I'm going to have to take steps."

"Take steps?  You mean, no more rolling stone?"

Then, in a dramatic reversal of roles, Zeus becomes victim of his own punishment.  He either comes up with a new punishment, lets Sisyphus off for good behavior, or moves Sisyphus to a place where the rock can do no damage, wherever if veers off course.

Exaggerated as this example may be, it shows the effect of momentum on story.  In this scenario, we have the physical momentum of the falling rock, the psychological momentum of an endless, boring task, and the adjunct physical momentum of the rock getting away from Sisyphus, who is justifiably drowsy on the job.

You being you, the story might also have Sisyphus using his iPhone to text as he rolls his rock upward, and/or some studio stuntman attempting to sell Sisyphus a prop rock, which would not be nearly so heavy as the original, nor would it gather as much momentum as it rolled down the hill, nor would it task Sisyphus so much (sciatica, you know, or perhaps the occasional touch of lumbago).

You being you, momentum is not the mere mixture of mass times speed, it is the direction of a considerable force against a considerable target, with considerable potential for mischief, the mischief of things getting away from their intended destiny, creating--how did you put it earlier?--collateral waves of supportive movement, along with direct opposition or the defensive responses of collateral characters and their own agendas.