What are the main contents and knowledge points of the junior high school physics pulley group?

For example, a fixed pulley and a movable pulley of the pulley group, the purpose of using the pulley block is to save labor, because the fixed pulley alone can not achieve the effect of labor-saving. So all the problems are on the moving pulley, you don't have to worry about the pulley, if you use a moving pulley, then its tensile force is the object 1 2, but it should be noted that the direction of the rope head is upward, at this time there are two ropes on both sides of the movable pulley, and so on, as long as there are a few ropes on the movable pulley, the four tie force is 1 4, it should be noted that if the fixed end of the rope is on the movable pulley, then it is necessary to count one more tie. The formula is, pull force = (gravity number of rope ties) + friction force, the length of the rope head falling or rising = the height of the object rising, multiplied by, the number of rope ties.

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The knowledge points of physics pulley in junior high school are as follows:

1. The pulley is a small wheel with a groove around the perimeter that can rotate around the shaft. A pulley is a simple machine that can rotate around a central axis consisting of a grooved disc that rotates around the central axis and a flexible rope (rope, tape, steel cable, chain, etc.) that crosses the disc.

2. Fixed pulley: When the plastic pulley bearing uses the pulley, the pulley with the fixed position of the shaft is called the fixed pulley.

3. The fixed pulley is essentially an equal arm lever, which is not labor-saving, but can change the direction of force. The power arm and resistance arm of the lever are respectively the radius of the pulley, and since the radius is equal, the power arm is equal to the resistance arm, and the lever is neither labor-saving nor labor-intensive.

4. When in use, the position of the pulley is fixed; The fixed pulley is essentially an equal-arm lever, which is effortless and effortless, but can change the direction of the force. The power arm and resistance arm of the lever are respectively the radius of the pulley, and since the radius is equal, the power arm is equal to the resistance arm, and the lever is neither labor-saving nor labor-intensive.

5. According to whether the position of the central axis of the pulley moves, the pulley can be divided into "fixed pulley" and "movable pulley"; The central axis of the fixed pulley is fixed, and the central axis of the movable pulley can be moved, each with its own advantages and disadvantages. And the fixed pulley and the movable pulley are assembled together to form a pulley block, which not only saves force but also can change the direction of force.

Pulley divides pulley and fixed pulley, 1, fixed pulley:

Definition: A pulley with a fixed shaft in the middle.

Substance: The essence of the fixed pulley is: it is equivalent to an equal-arm lever.

Features: The use of fixed pulleys can not save effort but can change the direction of power.

Description: For the ideal fixed pulley (excluding the friction between the axles) f=g the distance of movement at the free end of the rope s (or speed vf).

The distance h, or velocity vg, traveled by the weight

2. Movable pulley:

Definition: A pulley whose shaft moves with an object is called a movable pulley.

Essence: The essence of the movable pulley is: the power arm is a labor-saving lever twice that of the resistance arm.

Features: The use of movable pulleys can save half of the force, but it cannot change the direction of power.

Description: The ideal movable pulley (excluding the friction between the shafts and the gravity of the movable pulley) is: f=1 2g; s=

2H3, pulley block.

Definition: fixed pulley and movable pulley are combined to form a pulley group.

Features: The use of pulley block can not only save effort but also change the direction of power Ideal pulley block (excluding the friction between the wheel axles and the gravity of the movable pulley) tensile force f=1 ng; s=nh

Ignoring only the friction between the axles, the tensile force f=

1 n(g object + g motion); s=

NH Description: Method of assembling pulley block: First, find the number of strands of the rope according to the formula N=(G object + G movement) F. Then according to the principle of "odd and even". Assemble the pulley according to the specific requirements of the topic.

Summary of junior high school physics pulley knowledge points:

1. Definition: A pulley is a small wheel with a groove around the perimeter that can rotate around the axis; 2. Types of pulleys: fixed pulleys, movable pulleys, pulley groups; 3. Fixed pulley:

When using pulley, the pulley with fixed position of the shaft is called the fixed pulley, and the fixed pulley is essentially an equal-arm lever, which is not labor-saving, but can change the direction of force; 4. Movable pulley: the pulley that the position of the shaft moves with the pulled object is called the movable pulley, and the movable pulley is essentially the lever that the power arm is twice the resistance arm, saving 1 2 force and 1 times more distance; 5. Pulley group: The pulley group composed of fixed pulley and movable pulley can not only save effort but also change the direction of force, (when the pulley block is placed vertically) s=nh f=g total n (excluding friction) where s:

The distance the end of the rope moves.

The fixed pulley is not labor-saving, the moving pulley saves half of the force, and the pulley group is suspended by several sections of rope, and the tensile force used is a fraction of the weight of the matter

1. Definition: It is formed by matching a plurality of fixed pulleys and movable pulleys.

2. Features: It can save effort, and it can also change the direction of force. When using a pulley block, there are several sections of rope that hang the object, and the force used to lift the object is a fraction of the weight of the object, i.e. f = (1 n)*g object (condition: excluding the pulley, rope weight and friction).

Note: If the weight of the movable pulley is not ignored, then: f = (1 n) * (g object + g slip).

3. The relationship between the distance S of the dynamic movement and the distance h of the moving weight is: when the pulley block is used, the pulley block hangs the object with n sections of rope, and the distance moved by the force used to lift the object is n times the distance of the object moved, that is, S=nh. This is shown in the figure below.

n indicates the rope that bears the weight of the object. number of segments).

4. The relationship between the velocity at the end of the rope and the velocity of the object rising: v rope = nv object.

As long as you complete your daily plans and small goals in this way, you will be able to cope with your new learning and achieve your long-term goals. The pulley set of key physics junior high school knowledge points provided by the physics network provides you, I wish you a happy study!

Assembly of pulley blocks:

1) According to the relationship, find the number of segments n of the rope on the moving pulley;

2) determine the number of movable pulleys;

3) According to the requirements of the force direction, determine the number of fixed pulleys.

The principle of determining the number of fixed pulleys is: a movable pulley should be equipped with a fixed pulley, when the movable pulley is an even number of ropes, a fixed pulley can be reduced, but if it is required to change the direction of force action, a fixed pulley should be added. After determining the number of moving and fixed pulleys, the connection of the rope should follow the rule of "odd tethering and even tethering", and the pulleys should be wound from the inside to the outside.

The physical knowledge points of the pulley block are as follows:

1.Pulleys are divided into: fixed pulleys, movable pulleys, and pulley groups.

2.The movable pulley and the fixed pulley are combined together to form a pulley group. It is characterized by labor-saving, the ability to change the direction of force, and the distance.

3.pulleys in which the shaft can move with the object; In essence, the power arm is twice the labor-saving lever of the resistance arm; The characteristics of the old spike are labor-saving and distance-consuming, and the direction of the force cannot be changed.

4.pulleys with fixed shafts; The essence is an equal-arm lever; It is characterized by the fact that it does not save effort or distance, and the direction of force can be changed.

Data Extension:

The pulley system is a simple machine assembled by multiple movable pulleys and fixed pulleys. It is possible to save effort or change the direction of force.

The earliest painting of the pulley appears in an Assyrian relief in the 8th century BC. This relief shows a very simple pulley that can only change the direction of the force, the main purpose is to facilitate the application of force, and does not give any mechanical benefit. In China, the drawing of pulley devices first appeared in the Han Dynasty portrait bricks and pottery well molds.

The ancient Greeks classified pulleys as simple machinery. As early as 400 BC, the ancient Greeks already knew how to use a compound pulley. Around 330 B.C., Aristotle's eighteenth problem, in his book "Mechanical Problems", was devoted to the "compound pulley" system, and Archimedes contributed a lot of knowledge about simple machinery, explaining in detail the kinematic theory of pulleys.

It is said that Archimedes once used a duplex pulley alone to pull a large sea ship full of goods and passengers, <>

In the first century, Alessandro analyzed and wrote the theory of compound pulleys, proving that the ratio of load to force is equal to the number of rope segments that bear the load, known as the "pulley principle".

In 1608, in his Mathematical Collection, the Dutch physicist Simon Steffen showed that the ratio of the length of the path of the carriage between the force applied and the load in a pulley system was equal to the inverse ratio between the force and the load. This is the principle of virtual work of the prototype.