Walter Lewin shows how inertia works using a hanging block and two strings—one above and one below the block. Inertia is an object’s resistance to changes in its motion. When you pull the bottom string slowly, the force is transferred through the block, and the top string breaks because it takes the full load. But if you pull the bottom string quickly, the block doesn’t move right away due to its inertia, and the bottom string snaps instead. This shows that heavier objects resist sudden motion m