You are not alone in a lack of understanding about the nature of inertia. Physicists do not know why objects resist acceleration — why objects push back when pushed. They do not know for sure why your head snaps back when your car speeds up. Inertia “just is.” Also, contrary to popular assumption, scientists don’t understand the mechanism that causes matter to attract itself — the force of gravity that makes objects fall to the ground. To be sure, scientists have painstakingly measured the rates of fall and resistance, and so we can build all sorts of technology that work flawlessly within the equations of these everyday forces. But we do not know why these forces work the way they do.