[HPsquared]

I like the friction analogy but would have it on an inclined plane to account for the energy produced/consumed once it's started. For example a task which provides its own motivation (once started) would be downhill and may only need an initial push to overcome static friction, then might be self-sustaining. On the other hand, an unrelentingly unpleasant task would be "uphill" and require constant willpower (cf. mechanical power) to finish.

[dhimes]

Unless you just think of it like you are the person applying the "force" (to get something done), then the analogy is that you have to keep applying it but less once you get into motion.

[HPsquared]

Force of will, perhaps.