This document discusses the concept of inertia. It defines inertia as an object's tendency to remain at rest or in motion unless acted upon by an external force. It explains that inertia is influenced by mass, with more massive objects having greater inertia. Examples are given of how inertia causes objects and passengers in moving vehicles to continue moving when motion stops suddenly without external forces like seatbelts intervening. Safety measures in vehicles are described that counteract the negative effects of inertia during sudden stops.