There's a good reason for the law enforcing seatbelt usage inmany states.
很多国家法律规定使用安全带是有原因的。
Seatbelts saved nearly 12,000 lives in the year 2000.
2000年，安全带的使用挽救了近1200条生命。
In this Moment of Science, we take a look inside thesesimple but highly useful machines.
让我们通过科学一颗来了解一下这个简单但又十分有用的装置吧。
A seatbelt consists of a belt made of flexible webbing and a retractor device.
安全带是由弹性带和一个牵开器装置构成的。
The retractor, usuallylocated inside a plastic housing above the passenger's outer shoulder, consists of a spool aroundwhich the belt winds, and a spring attached to the spool to keep the webbing taut.
牵开器通常位于乘客外肩上方的塑料罩里，由一个缠绕弹性带的线轴和一个依附在线轴上使弹性带拉紧的弹簧构成。
When you pull a seatbelt across your chest and pelvis, the spool spins counter-clockwise,untwisting the spring.
当你拉开安全带绕过你的胸和盆骨时，线轴就会逆时针旋转，松开弹簧。
Since the spring wants to return to its coiled position, when you let go ofthe belt the spring causes the spool to spin clockwise, reeling in the slack.
当你解开安全带的时候，弹簧就会通过自身回力使线轴顺时针旋转，收紧安全带。
The most important part of a seat belt is the spool's locking mechanism—a device that makes thebelt hold tight in the unfortunate event of a crash.
安全带最重要的部分就是线轴的预收紧装置，在发生车祸的时候可以使安全带收紧，
The locking mechanism is activated either bythe car's movement or by the belt's movement.
汽车运动和安全带运动都会触发预收紧装置。
In car-activated systems, when the car stops suddenly a weighted pendulum swings forward,causing a metal bar to jam into a toothed gear attached to the spool.
在汽车启动触发的情况下，汽车突然停下时，一个加重的摆锤会向前摆动，金属棒就会卡进依附在线轴的齿轮里。
Unable to unwind anyfurther, the belt holds the passenger tightly in place.
安全带再无法被进一步放开的情况下就会紧紧的将乘客束缚在座位上。
In belt-activated systems, centrifugal force, cause by a sudden jerking of the belt, causes a leverattached to the spool to move outward.
在安全带激活系统中，由安全带突然震动造成的离心力会使线轴上的杠杆向外移动。
The extended lever activates a device that catches thetoothed gear attached to the spool, stopping its spinning motion.
延长的杠杆会触发线轴上与齿轮相连的一个装置，从而使轴芯停止转动。
In both systems, the point is totighten the belt so that its stopping power is spread across the sturdiest parts of your body.
这两种情况的关键就是要收紧安全带，这样安全带的收力才会传递到身体中最强壮的部分。
