Inertia is the resistance to change of motion. When in a car crash, the car has an external force applied by the thing they hit, like another car or a tree, meaning it will stop.
The person sitting in the seat without a seat belt will continue to move with the same velocity untill they smash into whatver is infront of them ie. the back of the front seat if they are sitting in the back.
The back of the seat infront of the person applies the force for a person to stop moving, but ofcourse the person has just had a high impact collision with a hard seat so they are probably uncouncious.
Wear your seat belt kids, because it will apply the neccesary force to stop you flying foward and it will save your life
