Newton's First Law of Motion (Law of Inertia): An object at rest stays at rest, and an object in motion continues at a constant speed in a straight line unless acted on by an external force.
Example: A book on a table remains still until someone lifts or pushes it. If you slide it, it eventually stops due to friction, an external force.
Explanation: The book's behavior shows inertia. It stays at rest without a force and only moves when you apply one. When sliding, it stops because of friction, demonstrating that objects resist changes in their state of motion unless acted upon by an external force. This illustrates Newton's First Law clearly.