Inertia

Inertia is the tendency of an object to remain in its current state—either at rest or moving at a constant speed in a straight line—unless acted upon by an external force. Essentially, objects resist changes to their motion. For example, a stationary book stays still until you push it, and a moving car will continue to roll unless brakes or obstacles slow it down. This property is fundamental in physics and explains why we need force to start or stop motion, ensuring predictable behavior of objects in our everyday environment.