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A driver or passenger travelling in a car is moving at the same speed
as the car. If the car suddenly stops, the body of the rider inside
will keep moving forward at the same speed. This demonstrates inertia
- the tendency of a moving object to keep moving, or of a stationary
object to remain at rest. An inertia-reel seat belt works on the same
principle. Its mechanism includes a pendulum, which hangs vertically
under ordinary driving conditions. If the car should suddenly stop,
however, it swings forward, and a locking lever resting on the
pendulum is released. The lever engages a toothed ratchet that locks
the shaft around which the belt is wound. The locked seat belt then
prevents the body from being flung forward.
When the seat belt is fastened, it winds out from the reel against
slight tension from a spring. This keeps it taut during normal
traveling, but allows enough free movement for a driver to reach
forward as necessary. If the driver tugs on the belt abruptly while
winding it out, the locking mechanism will engage and stop the action
of the spring. Slackening the belt releases the spring and the
locking lever.
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