To provide an unbalanced force to decelerate the passenger's forward motion.
Explanation
The function of a safety belt is a direct application of Newton’s First Law of Motion (Law of Inertia). This law states that an object in motion remains in motion with the same speed and in the same direction unless acted upon by an unbalanced external force.
Detailed Analysis:
- When a moving vehicle is involved in a collision or brakes suddenly, the vehicle decelerates rapidly. However, due to the inertia of motion, the passengers continue to move forward at the vehicle's original speed.
- If no external force stops them, the passengers would collide with the interior parts of the car (such as the dashboard or windshield).
- The safety belt provides the necessary unbalanced external force against the passenger's body. This force acts opposite to the direction of motion, decelerating the passenger and preventing them from being thrown forward.
- To increase the momentum of the passenger during a collision. is incorrect: The safety belt does not increase momentum; it helps reduce the passenger's momentum to zero safely.
- To reduce the friction between the passenger and the seat. is incorrect: While friction exists, the primary mechanism for stopping the passenger is the tension in the belt, not the friction with the seat.
- To prevent the passenger from moving backwards when the vehicle accelerates. is incorrect: This refers to the inertia of rest (feeling pushed backward when a car accelerates). This is primarily countered by the seat's backrest, not the safety belt.
Key Takeaway:
Safety belts provide the external force required to overcome the inertia of motion during a collision, thereby decelerating the passenger along with the vehicle.