Eli5: How/why do seatbelt lock and why does pulling it back unlock it?
In: 1
What you are discussing is the inertia reel seatbelt mechanism.
This is a small mechanical element within the seatbelt that is designed to let you wind and unwind the seatbelt at a normal speed to put it on and take it off, but in the event of a very sudden jolt like a crash it should lock up and hold you in place.
This works by using a force called inertia.
You know when your sit in a roundabout in a playground, when it moves slowly it is easy to sit normally in place, but as it is spun faster it becomes harder to stay put and you start to get thrown out towards the power edge of the roundabout? This is the force that makes the seatbelt lock. Inside the housing for the belt there is a little weight that spins round with a spring holding it in place. When you pull out the seatbelt slowly the spring holds the weight in place and it moves freely. When it moves quickly the weight sounds round fast enough to overcome the spring and move outwards like the person getting thrown off the roundabout, and this weight moving out causes the mechanism to lock in place.
The little wiggle you need to free it again after it locks is just moving the mechanism enough to allow the weight to reset and be pushed back into place by the spring, rather than remaining caught against the locking catch.
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