Non-Stretching Seatbelt

The task of the seatbelt is to stop you with the car so that your stopping distance is probably 4 or 5 times greater than if you had no seatbelt. A crash which stops the car and driver must take away all its kinetic energy, and the work-energy principle then dictates that a longer stopping distance decreases the impact force. For the example car crash scenario the stopping distance is one foot, the force on a 160 lb driver is about 4800 lb or 2.4 tons, and the deceleration about 30 g's. A moderate amount of stretch in the seatbelts will reduce the average impact force.

Stretching Seatbelt

A moderate amount of stretch in a seatbelt harness can extend the stopping distance and reduce the average impact force on the driver compared to a non-stretching harness. If the belt stretched 0.5 ft in the example car crash scenario, it would reduce the deceleration to 20 g's and the average impact force to 3200 lb compared to 30 g's and 4800 lb for a non-stretching seatbelt. Either a stretching or non-stretching seat belt reduces the impact force compared to no seatbelt.

No Seatbelt!

With no seatbelt to stop the driver with the car, the driver flies free until stopped suddenly by impact on the steering column, windshield, etc. The stopping distance is estimated to be about one fifth of that with a seatbelt, causing the average impact force to be about five times as great. The work done to stop the driver is equal to the average impact force on the driver times the distance traveled in stopping. A crash which stops the car and driver must take away all its kinetic energy, and the work-energy principle then dictates that a shorter stopping distance increases the impact force.

How far will the driver fly while the car is stopping?

When would you be better off not wearing your seatbelt?

Example car crash scenario

With no seatbelt to stop the driver with the car, the driver flies free until stopped suddenly by impact on the steering column, windshield, etc. If the distance to impact is greater than 2 ft, the car will be at rest or even bouncing back when the driver strikes it at 30 miles/hr.

When would you be better off not wearing your seatbelt?

Example car crash scenario