Inertial Frame

An inertial frame is a perspective from which an observer measures motion. In such a frame, objects that are not experiencing any force move at constant speeds or stay still. For example, if you're in a smoothly moving train, you can consider the train an inertial frame, as everything inside moves as though at rest. However, if you were inside a car that is accelerating or turning, it would not be an inertial frame because objects would appear to move relative to you without any apparent force acting on them. In essence, it's a stable, non-accelerating environment for studying motion.