"Pulley" Simple Machinery PPT (Lesson 2 Pulley Set)

Description

"Pulley" Simple Machinery PPT (Lesson 2 Pulley Set)

Part One: Knowledge Management

pulley block

Definition: The pulley block is composed of several fixed pulleys and movable pulleys.

Function: Using a pulley block can achieve the purpose of saving effort and changing the direction of force, but it requires more distance.

Rules: (1) Disregarding friction and the gravity of the moving pulley: F = 1nG object;

(2) Excluding friction: F = 1n (G object + G motion).

Pulley PPT, part 2: classification and exploration

Type one: Characteristics of pulley block

(2018•Chenzhou, Hunan) The purpose that can be achieved by using a pulley block is ( )

A. Saves effort and can definitely change the direction of force

B. Effort-saving but impossible to change the direction of force

C． Save effort and distance

D. Can save effort and change the direction of force

[Analysis] The advantage of a fixed pulley is to change the direction of force. The advantage of a movable pulley is that it saves effort, but cannot change the direction of force. A pulley set composed of a fixed pulley and a movable pulley can save effort and change the direction of force. So choose D.

[Enlightenment] This question examines the characteristics of the pulley block. We know that labor-saving machines must consume distance. There is no machine that saves both labor and distance.

Type 2: Pulley block winding problem

(2018•Xingtai, Hebei) As shown in the picture is an unassembled pulley block. Please draw the rope winding method of the pulley block in the picture. It is required to use this pulley block with the least effort when lifting heavy objects.

[Analysis] The rope is wound in sequence starting from the hook of the driven pulley. Finally, three strands of rope bear the weight of the object. This is the most labor-saving winding method. as the picture shows.

[Enlightenment] The labor saving of the pulley block depends on the number of strands of the rope that bears the weight of the object, that is, how many strands of rope are connected to the moving pulley. The more strands, the more labor-saving it is. In the pulley block, the more strands of rope that go around the movable pulley, the easier it will be, that is, F = 1nG.

[Method summary] (1) The most labor-saving method of winding the pulley block: first tie the rope to the fixed hook of the movable pulley, then go around the fixed pulley, and then around the movable pulley.

(2) There are two ways to wind the pulley block rope:

First, the rope is first tied to the fixed hook of the fixed pulley, then goes around the movable pulley below and then up to the fixed pulley, and is wound repeatedly in sequence. This winding method has an even number of rope sections to bear the weight of the object;

Second, the rope is first tied to the fixed hook of the movable pulley, then goes around the fixed pulley above and then downwards, and is wound repeatedly in sequence. This winding method has an odd number of rope sections to bear the weight of the object.

Pulley PPT, the third part: evaluation in class

1. (Corresponding Example 1) (2018·Tongcheng, Anhui) The more comprehensive description of the pulley block is ( )

A． It definitely saves effort and can change the direction of force.

B. It must save effort, but it cannot change the direction of force

C． Sometimes it can save effort and change the direction of force; sometimes it can save effort but cannot change the direction of force.

D. Using a pulley set will definitely save effort

[Analysis] As shown in Figure B, the pulley block has three strands of rope, and the direction of the force is upward, so it can save effort but cannot change the direction of the force, so A does not meet the meaning of the question. As shown in Figure A, the pulley block has two ropes, and the direction of the force is downward, so it can save effort and change the direction of the force, so B does not meet the meaning of the question. The pulley set in Figure A can save effort and can change the direction of the force; the pulley set in Figure B can save effort but cannot change the direction of the force, so C is consistent with the meaning of the question. If the gravity of the lifted object is much smaller than the gravity of the movable pulley, then using the pulley block cannot save effort, so D is wrong. So choose C.

2. (Corresponding Example 2) (2018·Tongliao, Inner Mongolia) Use the same pulleys to assemble the pulley set as shown in the figure. When using them to lift the same object, the most labor-saving method of winding the rope is ( )

[Analysis] The use of two pulleys in item A is equivalent to a movable pulley, which can save half the force, and the pulling force F=12G; the use of two pulleys in item B is equivalent to a fixed pulley, which cannot save effort, and the pulling force F=G; The two pulleys in item C form a pulley block, the number of rope strands to bear the weight of the object is n = 3, and the tension force used is F = 13G; the two pulleys in item D form a pulley block, the number of rope strands to bear the weight of the object is n = 2, and the tension force used is F = 12G. It can be seen that the most labor-saving rope wrapping method when using them to lift the same object is C. So choose C.

3． (Corresponding example 1) (Nanchong, Sichuan) As shown in the figure, under the action of the pulley group, object A with a weight of 300 N rises at a constant speed of 0.2 m/s. The movable pulley weighs 60 N. Excluding the rope weight and friction, then The tension F at the free end of the rope is ____N, and the power P of work done by the tension F is ____W.

Pulley PPT, part 4: layered operations

1. (Corresponding example 1) (2018·Guangyuan, Sichuan) Use the tool as shown in the figure to lift the same weight G, (excluding friction, excluding the weight of ropes, pulleys and levers) the most labor-saving thing is ( )

2. (Corresponding example 1) (2018•Suzhou, Jiangsu) As shown in the figure, the hanging basket weighs 400 N, the total weight of the movable pulley is 50 N, the fixed pulley weighs 40 N, the human gravity is 600 N, and the person is pulling the rope in the hanging basket When not moving, people need to exert force ( )

A． 218N

B. 220N

C． 210N

D. 236 N

3． (Corresponding examples 1 and 2) (2018·Jincheng, Shanxi) A student used the device in the picture to explore the pulley block. The object weighs 30 N. Which of the following statements is wrong ( )

A． The pulley block consists of a fixed pulley and a movable pulley, which can save labor and change the direction of force.

B. If the weight of the rope, the weight of the pulley and friction are not considered, the pulling force must be at least 10 N to lift the heavy object.

C． If the weight is lifted 3 cm, the free end of the rope needs to move 9 cm

D. In the experiment, the student lifted the object at a constant speed, and the pulling force used was greater than 10 N

[Analysis] The pulley set in the picture consists of a fixed pulley and a movable pulley. Using this pulley set can save labor, but the last rope goes around the movable pulley, which plays the role of bearing the weight of the object and cannot change the direction of the force, so A Wrong; if the weight of the rope, the weight of the pulley and friction are not taken into account, the pulling force is at least F=13G=13×30 N=10 N, so B is correct; according to the characteristics of the pulley set, it can be seen that the distance moved by the free end of the rope in the same time is Three times the moving distance of the object, the free end of the rope needs to move 9 cm, so C is correct; if the rope weight, pulley weight and friction are considered, the heavy object needs to be lifted, and the pulling force F=1n (G+G motion), then the pulling force used is greater than 10 N , so D is correct. Therefore choose A.

4. (Corresponding Example 1) (2018•Chuzhou, Anhui) As shown in the figure, object A on the horizontal ground moves in a straight line with uniform speed to the left under the action of pulling force F. It is known that the friction force on object A is 30 N, GA = 100 N, then the pulling force of the rope on A F′ = ____N, and the force acting on the free end of the rope F = ____N. (Excluding pulley weight and friction between ropes)

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Update Time: 2024-06-28

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