Sara is sitting in the back seat as her father drives at a constant speed.If Sara's father presses on the brakes quickly slowing the car down what slows Sara down?

When Sara's father presses the brakes and the car slows down quickly, Sara's body tends to continue moving forward due to inertia, a principle described by Newton's first law of motion. The seatbelt, if she is wearing one, helps to restrain her and slows her down along with the car. Without a seatbelt, she could lurch forward, experiencing a rapid deceleration relative to the car.