Final answer:
When someone slams on the brakes in a moving car, a person not wearing a seatbelt can be thrown forward due to their body's tendency to remain in motion. Wearing a seatbelt helps to bring the person to a stop along with the car, reducing the risk of injury.
Step-by-step explanation:
When someone slams on the brakes in a moving car, the car comes to a sudden stop. However, according to Newton's first law of motion, an object in motion tends to stay in motion unless acted upon by an external force. So, when a person in the car is not wearing a seatbelt, their body wants to continue moving forward at the same speed even after the car has stopped. As a result, the person can be thrown forward and potentially fly through the car window.
When a person wears a seatbelt, it applies a restraining force on them, bringing them to a halt along with the car. This decreases the time over which the person stops, reducing the force exerted on their body and minimizing the risk of injury.