Which comes back to the physics of it all. As University of Virginia politics Prof. Larry J. Sabato is fond of saying, the two most powerful forces in politics are inertia and gravity, in that order.

What you're actually feeling, though, is just the rock's inertia. The law of inertia says that something in motion will tend to stay in motion, and what's more, to move in a straight line.

What you're actually feeling, though, is just the rock's inertia. The law of inertia says that something in motion will tend to stay in motion, and what's more, to move in a straight line.