Without physics, baseball wouldn't be possible. But you don't need to understand the intricate ins and outs of Newton's Laws of Motion to comprehend The Art of Fielding. (Although if you're itching for a crash course in physics, we can help.) The inertia we're talking about here is the desire for things to remain the same. It's on the same coin as laziness, or at least on a similar coin in the same change purse. And just as baseball wouldn't be possible without inertia in physics (an object in motion tends to stay in motion, yada yada yada), the Art of Fielding wouldn't be possible without Henry's desire for sameness. Once that kid stops moving, it's hard to get him to start again.
Questions About Inertia
Why doesn't Henry want anything to change? What, exactly, does he want to keep the same? What actions does he take to prevent things from changing?
What other characters exhibit similar tendencies toward inertia? For example, why does Guert want to stay at Westish forever? Does Mike also intend to become a lifer?
Which characters are still the same at the end of the book as they are in the beginning? What about them, if anything, has changed?
Chew on This
Baseball as a sport rarely makes any major changes, so thematically it makes sense for The Art of Fielding to feel like one big never-ending, never-changing game of ball.
The only thing that progresses slower than baseball is academia, as evidenced by Affenlight's struggles to make Westish a greener campus. We have to note his lack of action was part of the problem, until Owen came along.