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Archimedes discovered the principle of lever leakage, that is, "when two heavy objects are in equilibrium, their distance from the fulcrum is inversely proportional to their weight." Archimedes' research on lever not only stays in theory, but also makes a series of inventions according to this principle. It is said that he once entered the water like a mast parked on the beach with the help of a lever and a pulley block.
In the battle to defend Syracuse from the Roman navy, Archimedes used the lever principle to make long-range and short-range catapults, and used them to launch various missiles and boulders to attack the enemy, which once made the Romans stay outside Syracuse for three years.
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. Namely: power× power arm = resistance× resistance. Mathematically expressed as F 1 L 1 = F2 L2. (L 1 means the power arm is disconnected, F2 means the resistance, and L2 means the resistance arm).
Lever balance: Hang equal weights at both ends of the weightless bar at the same distance from the fulcrum, and they will be balanced. Unequal weights are hung at both ends of the weightless bar at the same distance from the fulcrum, and the heavy end will tilt downward. Hang equal weights at both ends of the weightless bar at different distances from the fulcrum, and the far end will tilt downward.
As long as the position of the center of gravity remains unchanged, the function of one weight can be replaced by the function of several evenly distributed weights. When using a lever, in order to save labor, a lever with a power arm longer than a resistance arm should be used; If you want to save distance, you should use a lever with a force arm shorter than a resistance arm. Therefore, the use of levers can save manpower and distance.
However, if you want to save energy, you must move more distances; If you want to move a shorter distance, you must work harder. Saving energy and moving distance is impossible.
In addition, the lever can be divided into labor-saving lever, labor-saving lever and equal arm lever. These types of levers have the following characteristics:
1, labor-saving lever: that is, the power arm is larger than the resistance arm. Such as claw hammer, carpenter's pliers, unicycle, soda wrench, hay cutter and so on.
2, laborious lever: that is, the power arm is smaller than the resistance arm. Such as tweezers, fishing rods and scissors for hair cutting.
3. Equal arm lever: a lever that is neither labor-saving nor laborious. Power arm is equal to resistance arm and its mechanical benefit is equal to 1. Such as scales and crown blocks.