Inertia
Because you are decelerating sharply
When car was moving, passengers were also moving. When car suddenly stops, the moving passengers try to maintain their state of forward motion because of their inertia. so they move forward relative to their seats...
If you are in a fast moving car and the brakes are applied suddenly and hard, your body will continue moving in the forward direction.
Inertia
If all the passengers are properly buckled in and the car brakes suddenly to a stop, someone may spill their coffee.
Weight transfer. When a car brakes, its weight moves forward and causes the front brakes to do the majority of the work in stopping the car.
It keeps you from moving forward if the car is hit or stops suddenly.
Force of Inertia
friction helps the movement of the car by allowing it to move easily and smoothly. -------------- Friction between the tires and the road propels the car forward and steer it. Friction in the brakes helps control the motion.
This depends entirely what type of car this is but: There is a serious failure of brakes or other safety equipment.
inertia
maybe not in gear