Explain how a seatbelt reduces the force on the body during a car accident.

• A. It increases stopping time, reducing average force.
• B. It decreases the momentum change.
• C. It dissipates energy as heat more quickly.
• D. It reduces the momentum of the car.

Answer: (A)

The key idea is that reducing the force on the body comes from taking longer to come to a stop. A seatbelt holds you in place, so when the car crashes and everything else slows rapidly, your body doesn’t slam forward as hard. By keeping you restrained, the stopping time is lengthened, which lowers the deceleration you experience. Since force equals mass times acceleration, and acceleration is the change in velocity over time, a longer stopping time means a smaller average force on you, even though your forward momentum goes to zero. The belt doesn’t change how much momentum you lose, and energy is dissipated in multiple parts of the crash, but the main effect of the belt is to spread the stopping over a longer time, reducing the force.