Video narration
Newton’s First Law is about what happens when forces don’t change an object’s motion. It says: an object at rest stays at rest, and an object in motion stays in motion, unless acted on by an unbalanced external force. That tendency to resist changes in motion is called inertia. So if a book is sitting on a desk, it won’t start moving by itself—you need a push. And if something is already sliding, it will keep going in the same straight line at the same speed if there’s no unbalanced force. Here’s the key idea: balanced forces cancel out, so the overall, or net, force is zero. Unbalanced forces don’t cancel, so the net force is not zero—and that’s when motion changes, like starting, stopping, speeding up, slowing down, or turning.