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MOTION · B1 · 002 · Inertia

When the car stops, you don't.

Inertia: things keep doing what they're doing until a force changes them. Your seatbelt uses inertia to know when to lock.

Try itThe lock inside the belt reel

Brake harder or park on a steeper hill. The ball can't tell braking from tilting. Once it feels a tilt past the lock point, it rolls out of its cup and the belt locks.

Braking 0 g

Hill slope (nose down) 0°

Ball feels a tilt of–

Lock point in this sim–

BeltFREE

Belt holds back (75 kg)–

NumbersHow big is it?

Rules and real places. The lock point in the sim is an example value; every belt model is a bit different.

0.7 g

US rules (FMVSS 209) say the belt must lock before it pulls out 1 inch at this deceleration.

≈ 740 N

What the belt holds back in a 1 g emergency stop if you weigh 75 kg (75 kg × 9.81 m/s²). Your own body weight, pushing forward.

17.5°

Filbert Street, San Francisco (31.5% grade). Steep enough to tip the ball in many belts. The same rules say no lock up to 15°.

HowWhat happens, step by step

The car brakes and the ball keeps going.
A small steel ball sits in a cup under the reel. Inertia carries it forward.
The ball rolls up the cup and lifts a lever.
It only gets out once the braking (or the slope) is strong enough.
The lever catches a tooth on the spool.
The reel can't turn, so the belt can't come out. You stay in your seat.

The second lock: small weights spin with the spool. Yank the belt fast and they swing outward and catch a tooth, even if the car isn't moving.

Check3 quick questions

Bonus

The same ball explains why your belt sometimes won't come out on a steep hill. The car is tilted, so the ball rolls out as if you were braking.