Levers are divided into labor-saving lever, labor-saving lever and equal arm lever. The lever principle is also called "lever balance condition". In order to balance the lever, the two moments (product of force and arm) acting on the lever must be equal.
That is, power× power arm = resistance× resistance arm, which is expressed by algebraic expression as F 1 L 1 = F2 L2. As can be seen from the above formula, to balance the lever, the power arm is several times that of the resistance arm, and the resistance is several times that of the power.
Proposal in principle
Archimedes, an ancient Greek scientist, put forward the lever principle in his book The Balance of Plane Figures.
Mozi observed the lever in the Warring States Period, and said in "Mozi Jing Shuo Xia" that "the balance is heavier than its edge, it will be hit, and the weight will be similar." Balance, short standard length. If the two added weights are similar, bid and win the bid. "
These two articles elaborated the balance of leverage very comprehensively. There are equal arms and unequal arms inside; Some change the weight of both ends to make it biased, and some change the length of the two arms to make it biased.
Archimedes has a long-standing famous saying: "Give me a fulcrum and I can pry up the whole earth!" " "This sentence means the principle of leverage.
Archimedes first regarded some empirical knowledge in the practical application of lever as "self-evident axioms", and then from these axioms, he used geometry and strict logical argumentation to get the principle of lever.
Knowledge expansion: human lever
Lever is indispensable in almost every machine, even in the human body, there are many levers at work. Picking up a thing, bending down, or even raising your toes are all levers of the human body. Understanding the lever of human body can not only increase physical knowledge, but also learn a lot of physiological knowledge.
Most of them are laborious levers, and a few are equal-arm labor-saving levers.
Nodding or looking up depends on the action of the lever. The fulcrum of the lever is at the top of the spine, and there are muscles before and after the fulcrum. The weight of the head is resistance. The muscles before and after the fulcrum cooperate, some contract and some lengthen, forming a bow and looking up. As can be seen from the picture, it is more labor-saving to bow your head than to raise your head.