How does a seat belt help a car passenger in the event of an accident?
When a car is in motion, the passengers seating in it are also moving at the same speed. If the car is moving at a speed of 70kmph (kilometres per hour), for example, the passengers are also moving at the same speed though they are not aware of it.
Now, if the car crashes, its speed rapidly changes from 70kmph to 0kmph. The passengers are still moving at 70kmph at the time of impact, in accordance with Newton's first law of motion which states that "a body in motion tends to keep moving until an external force is applied to stop it".
An external force has stopped the motion of the car but not that of the passengers. They will hurtle forward at 70kmph and hit the dashboard or crash through the windshield. But not if they're wearing seat belts.
Firstly, a seat belt, being slightly elastic, stretches to slow down the abrupt halt of the passenger. It also secures the passenger to the seat thereby stopping him/her from being thrown forward. Thirdly, it spreads the force of impact over the stronger parts of the body like the lap (pelvis) rib cage and shoulders.
So wearing a seat belt can prevent grievous hurt or even death, in an accident.
