Conservation of Energy in Physics

Energy is never created and never destroyed, so every problem is really just a question of where it went. Draw a boundary and the bookkeeping collapses to two cases. Either nothing crosses the boundary, in which case the total inside is frozen and only its form can change, or something does cross it, in which case the change inside equals what came through. Friction deserves a warning of its own, because it never gives energy back. It turns mechanical energy into internal energy and leaves the bill for you to pay, which is why the friction term always appears on the outgoing side of the equation. The blocks give the two models, the friction cases, and the two speed results they most often produce.