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/s
^{2} = 1474 m/s^{2} = 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/s
^{2} = 197 m/s^{2} = 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.

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