卷尺的“内芯”，里面有根小的一截钢片，作用就跟弹簧差不多。也可以这样想，就是玩具手枪扳机上面的那个，用来“打击”子弹的那跟小钢丝，拉出来的时候使它受力弯曲，松开后外力（指手拉的力，而摩擦力较小，小于了小钢丝的弹力）消失，所以再自动弹回。 而卷尺上最前面有个折叠的那一部分，这时候就起作用了，使卷尺卡在外面，使它不会完全缩进去。 一开始他是松的，拉出来后他就变紧了，松手后它应该就有个复原的过程，所以再自动弹回。 就是他最开始里面就是松的，拉的时候就紧了，之间有个弹力，再松开自然就有弹回去。
Steel shield rubber and plastic tape measure 5m The main type of tape measure is steel tape, followed by fiber tape, which is the tape that everyone often sees. Many people say it is a cloth ruler, and there is also a waist measurement ruler (tailor ruler/clothing ruler belong to this category). In the southern part of my country and Hong Kong, the tape measure is usually called a soft ruler or a pull ruler. The material is PVC plastic and glass fiber. The glass fiber can prevent the tape from being stretched during use. The tape measure can be rolled up because the tape measure is equipped with a spring. When the tape measure is pulled out to measure the length, the length of the ruler and the spring is actually stretched. Once the measurement is completed, the spring inside the tape measure will automatically contract, and the ruler will follow the spring force. Shrink, so the tape will roll up. 1. The tape measure is too deformed and will not fail. Why didn’t it fail? The spring in the tape measure is very long, and it is completely relaxed to the complete tightening range. The entire stroke of the tape measure includes it, and there is margin at both ends, so the tape measure will not fail. Second, there is no difference in tension between a one-meter length and two-meter length; the spring of the tape measure is very thin, and the amount of deformation when tightening is very small, so the reaction force is very small, and no force change is felt when the rule is pulled. The “inner core” of the tape measure has a small piece of steel inside, which acts like a spring. You can also think of it this way. The small wire on the trigger of the toy pistol is used to “strike” the bullet. When it is pulled out, it will be bent under force, and the external force (refers to the force of the hand, and the friction is more Small, less than the elasticity of the small steel wire) disappears, so it automatically rebounds. And there is a folded part on the front of the tape measure, which works at this time, so that the tape measure is stuck outside so that it will not be completely retracted. He was loose at the beginning, but after pulling it out, he became tight. After letting go, it should have a recovery process, so it automatically rebounds. It’s just that the inside is loose at the beginning, and it becomes tight when pulled, and there is an elasticity between it, and when it is loosened, it will naturally spring back.