Why should we use a seatbelt?
Forces in Car Crashes
Example of Force on Car
Using seat belt makes it different
For the car crash scenario where a car stops in 1 foot from a speed of 30 mi/hr, what is the force on the driver? Assume a 160 lb (mass = 5 slugs) driver.
If firmly held in non-stretching seatbelt harness: Stopping distance 1 ft.
If not wearing seatbelt, stopping distance determined by nature of collision with windshield, steering column, etc. : stopping distance 0.2 ft.
| || |
- Deceleration = 4836 ft/s2 = 1474 m/s2 = 150 g's
- Force = 24068 lb = 107059 N = 12 tons!!
|No seatbelt! |
If seat belt harness stretches, increasing stopping distance by 50%: 1.5 ft.
| || |
- Deceleration = 645ft/s2 = 197 m/s2 = 20 g's
- Force = 3209 lb = 14274 N = 1.6 tons
|Stretching seatbelt |
These calculated numbers assume constant deceleration, and are therefore an estimate of the average force of impact.