He aquí por qué:

* inercia es la tendencia de un objeto a resistir los cambios en su movimiento. This means an object at rest wants to stay at rest, and an object in motion wants to stay in motion.

* When your car comes to a stop, the car itself stops, but your body, due to inertia, wants to continue moving forward.

This is why you feel pushed forward when the car brakes suddenly, and why you feel pushed back when the car accelerates quickly.