Physics pulley knowledge points in junior high school, physics pulley knowledge points in junior hig

The fixed pulley is essentially an equal-arm lever, which does not save force, 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. It is not labor-saving to pull the object through the fixed pulley.

With or without a fixed pulley, the spring dynamometer reads the same. It can be seen that the use of fixed pulleys does not save force but can change the direction of force. In many cases, changing the direction of the force will bring convenience to the work.

The movable pulley is essentially a lever that the power arm is twice that of the resistance arm, saving 1 2 force and 1 times more distance. The use of movable pulleys can save half the effort and consume the distance. This is because when using movable pulleys, the hook is suspended by two sections of rope, each of which bears only half the weight of the hook.

Although the use of movable pulley saves effort, the distance of power movement is greater than the distance of the hook code elevation, that is, it takes the distance. A pulley in which the position of the shaft moves with the object being pulled is called a movable pulley. It is a deformed unequal arm lever that saves half the force (without considering the gravity and friction of the pulley), but does not change the direction of the force.

The use of movable pulleys can save half the effort and consume the distance. This is because when using movable pulleys, the hook is suspended by two sections of rope, each of which bears only half the weight of the hook. Although the use of movable pulleys saves effort, the distance of power movement is twice the distance raised by the hook code, that is, it takes the distance.

The direction of the force cannot be changed. Moves as the object moves.

Characteristics of fixed pulley: it is not labor-saving, but it can change the direction of power. (Essentially an equiarm lever).

Characteristics of movable pulley: save half of the force, but can not change the direction of power, it costs distance. (The essence is that the power arm is twice the lever of the resistance arm).

Pulley: When using a pulley, the pulley block uses several lengths of rope to hang the object, and the force used to lift the object is a fraction of the weight of the object.

There are several knowledge points that need to be understood: f, s, n, g, hIt is the pulling force of the rope, the distance of the rope, the number of ropes that are lifted on the east pulley, the weight of the weight, and the height of the weight.

Fixed pulley: does not save force, but can change the direction of force.

Movable pulley: save 1 2 times the force, cost 2 times the distance, can not change the direction of the force.

Pulley group: Look at how many sections of rope carry the moving pulley, there are several sections of rope, the force is a fraction of a percent, pay attention to the moving pulley.

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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

The physical knowledge points of pulleys are as follows:

The pulley is an important mechanical simple machine in physics, which is mainly used to change the direction and magnitude of force. It consists of a wheel and ropes, chains, etc., that rotate around the wheel. The following are the detailed knowledge points about pulleys::

1. Types of pulleys

There are two types of pulleys: fixed pulleys and mobile pulleys, the former is fixed at one end, and the latter can be moved, which is commonly called a pulley.

2. The working principle of the pulley

The working principle of the pulley is to use the toughness of the bent rope or chain so that the external force is transmitted to the other end through the pulley while changing the direction and magnitude of the force. In the resting state, the tension of the pulley is equal but in a different direction; When the pulley is moving, the external force needs to overcome the friction force and the inertia of the pulley itself.

3. The mechanical formula of the pulley

The mechanical formulas of pulleys include "dynamic theorem", "Newton's second law", and "lever principle", among others. Among them, the dynamical theorem states that power is equal to force multiplied by velocity; Newton's second law describes the relationship between the net force f and the acceleration a of an object with mass m; The lever principle illustrates the amplification and contraction of force by using the proportional relationship between the arm and the force.

4. Pulley application field

Pulleys are widely used in various fields, such as construction engineering and sports training. In construction projects, pulleys can be used to lift heavy loads and reduce labor and time costs; In sports training, pulleys can be used to change the direction and magnitude of force to enhance muscle training.

, pulley precautions

When using the pulley, the following matters need to be noted: ensure the safety and stability of the pulley; Choose appropriate ropes, chains, and other materials; Avoid overloading the use of pulleys for pruning; Regularly inspect and maintain pulley equipment.

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 wrap the pulley from the inside to the outside.

Basic principle: the tension on a rope is equal everywhere, count how many ropes are wound on a pulley, the pulley is balanced by force, if there are two ropes at one end of the pulley and a rope at the other end, then this is the reason why the moving pulley saves half of the force. Other sets of pulleys, no matter how complex, can be explained by this principle.

Pulleys are simple machines used to lift heavy loads and save effort. The pulley is a small wheel with grooves around the ear and a groove around the edge, which can rotate around the axis. A simple machine consisting of a grooved head-gaze disc that rotates around the central axis and a flexible rope that crosses the disc is called a Tuanqin pulley.

Brother pulleys can remember two things: one is that when the pulley moves in a straight line at rest and at a uniform speed, the combined external force is 0; The tension of the same rope on the second pulley is equal, and all questions are OK. Don't remember the mess, it's useless, and it affects your judgment, because the outcome is different depending on the situation.

The essence of fixed pulley and movable pulley, the method of winding the rope of the pulley block, and the mechanical efficiency

There are basically two points:

Draw a plot for force analysis.

Interaction of forces.