1、 Definition and five elements of leverage
1. Definition: If a hard stick can rotate around a fixed point under the action of force, it is called a lever.
2. Five elements:
(1) Fulcrum: the point around which a lever can rotate.
(2) Power: The force that turns the lever.
(3) Resistance: The force that prevents the lever from turning.
(4) Power arm: the distance from the fulcrum to the power line.
(5) Resistance arm: the distance from the fulcrum to the resistance action line.
3. Definition of lever balance: under the action of force, if the lever is at rest or rotates at a constant speed around the fulcrum, we think the lever is balanced.
4. Equilibrium conditions:
Power × power arm=resistance × resistance arm, expressed by the formula $F_1l_1=F_2l_2 $.
5. Classification:
(1) Effort saving lever: the power arm is larger than the resistance arm, which saves effort but costs distance. Such as crowbar, claw hammer, hoe, etc.
(2) Effort lever: the power arm is smaller than the resistance arm, which is laborious but saves distance. Such as boom, sewing machine pedal, fishing rod, etc.
(3) Equal arm lever: the power arm is equal to the resistance arm, with no effort or effort. Such as balance, fixed pulley, etc.
2、 Examples of leverage
The correct one of the following statements is____
A. Only when the lever is stationary can it be called lever balance
B. When the lever is balanced, the fulcrum must be in the middle of the lever
C. Lever balance can be called when the lever is stationary in the horizontal position
D. The lever rotates at a constant speed, which can also be called lever balance
Answer: D
Analysis: Lever balance means that the lever is in a static state or rotating at a constant speed. When balancing, the fulcrum is not necessarily at the midpoint, and the lever is not necessarily at the horizontal position. Therefore, select D.