By Chris Deziel Updated Mar 24, 2022
Image courtesy of iStock.
Newton’s second law of motion states that the force applied to an object equals its mass times its acceleration (F = m·a). When you’re secured with a seat belt, the belt supplies the force necessary to decelerate you safely during a collision, preventing impact with the windshield or steering wheel.
When a vehicle accelerates, the seat exerts a force that brings your body along with the car. The heavier you are or the faster the car goes, the greater this force becomes. Upon stopping, your body continues moving until an opposing force acts on it. If the vehicle slows gradually, your legs can absorb this force. However, in a sudden stop—such as hitting an obstacle—your body’s deceleration exceeds what your limbs can handle.
For example, a 68‑kilogram (150‑pound) passenger traveling at 26.8 m/s (60 mph) would require a force of 364 N (≈1,800 lb) to stop within five seconds. If the car stops abruptly, the required force spikes to 1,822 N (≈9,000 lb). Without a seat belt, this force is transferred to the windshield or steering wheel, which can be fatal.
Modern seat belts include a shoulder harness to prevent the upper body from sliding forward when the car stops. Yet, even with this design, the head can move rearward as the torso recoils, leading to serious injuries. Airbags have been introduced to absorb forward momentum and distribute the stopping force over a larger area, reducing impact severity.
