"Lever" Simple Machine PPT (Lever Classification and Application in Lesson 2)

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"Lever" Simple Machine PPT (Lever Classification and Application in Lesson 2)

Part One: Knowledge Management

1. Classification of leverage

Labor-saving lever: the power arm is larger than the resistance arm, and the power is smaller than the resistance. Effortless but _____.

Effortless lever: The power arm is smaller than the resistance arm, and the power is greater than the resistance. Effortless but _____.

Equal-arm lever: The power arm is equal to the resistance arm, and power is equal to resistance. It saves neither effort nor distance.

Features: If the lever saves effort, it will increase distance; if it saves distance, it will increase effort.

2. Application of leverage

The balance is the application of _____ lever; the pliers are the application of _____ lever; the paddle is the application of _____ lever.

Leverage PPT, part 2: classification and exploration

Type One: Classification and Application of Leverage

(Shaoyang, Hunan) Among the simple machines shown in the figure, the labor-saving lever is ( )

[Analysis] When using a fishing rod, the power arm is smaller than the resistance arm, which is a labor-saving lever; when using a hammer, the power arm is larger than the resistance arm, which is a labor-saving lever; when using chopsticks, the power arm is smaller than the resistance arm, which is a labor-saving lever. Lever; During the use of tweezers, the power arm is smaller than the resistance arm, so it is a laborious lever. So choose B.

[Summary of rules] Leverage mainly includes the following types: ① Effortless lever, the power arm is larger than the resistance arm, which saves effort but costs distance; ② Effortless lever, the power arm is smaller than the resistance arm, which requires effort but saves distance; ③ Equal-arm lever, the power arm is equal to the resistance The arm saves neither distance nor effort.

Type 2: Application of Leverage Balance Conditions

(2018•Beijing) As shown in the figure, the mass of each hook code is the same and the lever is in a balanced state. Which of the following can cause the left end of the lever to sink ( )

A． Add a hook code on each side

B． The hook codes on both sides move outward one space each.

C. Add a hook code on the left and move two spaces outward on the right

D. Move two spaces outward on the left and add a hook code on the right

[Analysis] Suppose the gravity of a hook code is G, the length of one grid is L; add a hook code on each side, the product of the force on the left side and the moment arm is 3G×2L=6GL, and the product of the force on the right side and the moment arm is 2G×4L=8GL; then the right side > the left side, so the right end of the lever sinks, so A does not meet the meaning of the question. The hook codes on both sides move outward one space each. The product of the left force and the moment arm is 2G×3L=6GL, and the product of the right force and the moment arm is 1G×5L=5GL; then the left side > the right side, so the left end of the lever sinks, so B fits the meaning of the question. Add a hook code on the left and move two spaces outward on the right. The product of force and moment arm on the left is 3G×2L=6GL, and the product of force and moment arm on the right is 1G×6L=6GL; then the right side = left side, The lever is still balanced, so C does not meet the meaning of the question. Move the left side outward two spaces and add a hook code to the right side. The product of the force on the left side and the moment arm is 2G×4L=8GL, and the product of the force on the right side and the moment arm is 2G×4L=8GL; then the right side=left side, The lever is still balanced, so D does not satisfy the question. So choose B.

[Impression] This question examines the application of the lever balance condition. Whether the lever is balanced depends on whether the product of the force and the moment arm is equal. Only comparing the size of the force or the moment arm cannot give a correct result. Lever balance conditions are widely used. Using lever balance conditions, you can find out the size of force or moment arm, judge whether the lever is balanced, or judge the change in force, etc.

Leverage PPT, the third part: evaluation in class

1. (Corresponding Example 1) (2018·Qiqihar, Heilongjiang) The appliance shown in the picture, during normal use, is a laborious lever ( )

2. (Corresponding Example 1) (Yantai, Shandong) Tiptoe is a good aerobic exercise (as shown in the picture). It is simple and easy to learn, is not restricted by the venue, and is deeply loved by the masses. The basic model of tiptoe movement is the lever. The following relevant analysis is correct ( )

A． The heel is the fulcrum, the labor-saving lever

B． The heel is the fulcrum, the lever of effort

C. The place where the sole of the foot touches the ground is the fulcrum, a labor-saving lever.

D. The place where the sole of the foot touches the ground is the fulcrum, the lever of effort.

3. (Corresponding to Example 2) Two forces act on both ends of the lever to balance the lever, then ( )

A． The magnitude of these two forces must be equal

B． The moment arms of these two forces must be equal

C. The force with the longer arm must be smaller

D. The force with the longer arm must be greater

4. (Corresponding Example 1) (2018•Changsha, Hunan) Use the crane as shown in the figure to lift heavy objects. Which of the following statements is correct ( )

A． The crane is equivalent to a labor-saving lever

B． The crane is equivalent to an equal-arm lever

C. Using this crane can save distance

D. Using this crane can save effort

5. (Corresponding examples 1 and 2) (2018·Wuhan, Hubei) The relevant dimensions of a wheelbarrow carrying bricks are as shown in the figure. The total gravity G on the cart and the bricks is 980 N. The resistance on the ground is 120 N. The worker pushes the cart at a constant speed. When moving forward 10 m, which of the following statements is incorrect ( )

A． The wheelbarrow is equivalent to a labor-saving lever

B． The upward force of the human hand is 420 N

C. It will be more laborious to move the hand forward on the handlebar.

D. A person must do at least 1 200 J of work

Leverage PPT, part 4: layered operations

1. (Corresponding Example 1) (2018·Chengdu, Sichuan) Among the simple machines shown in the figure, the one that is a laborious lever is ( )

2. (Corresponding example 2) (2018•Linyi, Shandong) Xiao Ming uses a steelyard to weigh the mass of a fish (as shown in the picture). When the steelyard is balanced in a horizontal position, the weight pull line is pressed exactly on the 2 kg mark, using the measurement at hand Tool, the mass of the weight can be estimated to be approximately ( )

A． 5 g b. 50g

C. 500 g D. 5 000g

3. (Corresponding to Examples 1 and 2) (2018·Kunming, Yunnan) As shown in the figure, the lightweight lever OA can rotate around point O without friction. An object weighing 20 N is hung at point A, and a vertical object is added at point B. The force F, the lever is balanced in the horizontal position, and OB:AB=2:1, then F=____ N, it is a ____ lever.

4. (Corresponding Example 2) (2018·Liangshan Prefecture, Sichuan) As shown in the picture, an iron rod with uniform mass is folded into a right angle in the ratio of 4:3, and one end is fixed on the wall with a hinge. The iron rod can go around point O Rotating, the rod weighs 70 N. To keep the rod balanced at the position shown in the figure, the minimum force applied to end A is ____ N.

[Analysis] It can be seen from the lever in the figure that the maximum moment arm of the force acting on end A is OA. After the iron rod is folded into a right angle in the ratio of 4:3, the force is perpendicular to OA at this time, and the force F is the smallest, as shown in the figure. :

OA＝OB2＋BA2＝42＋32＝5,

It is known that the rod weighs 70 N, then the gravity of the OB part and the BA part are respectively: GOB=47×70 N=40 N, GBA=37×70 N=30 N,

From the leverage balance condition: GOB×12OB+GAB×OB=F×OA,

That is: 40 N × 12 × 4 + 30 N × 4 = F × 5, and the solution is F = 40 N.

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Update Time: 2024-10-03

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