Why did the boy and girl get "pushed" forward even though they weren't physically pushed? Why did they continue to move forward even after the car had stopped?
Newton's First Law tells us: an object in motion will stay in motion, and an object in rest will stay in rest (until acted on by an external force). This is also known as the concept of "inertia". When the car braked, the sudden force of friction between the cars' wheels and asphalt caused the passengers inside to be thrown forward from their relaxed positions. In this case, the friction is the external force that changed the passenger's "rest" into motion.