"Optimization Summary of this Chapter" Interaction PPT

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"Optimization Summary of this Chapter" Interaction PPT

Integration and improvement

Comparison of three types of properties

1. The difference and connection between gravity, elasticity and friction

type

Item Gravity Elasticity Friction

Causes: The attraction of the earth. Objects come into contact and undergo elastic deformation. Objects in contact squeeze each other and undergo relative motion or have a tendency to move relative to each other.

The size G=mg is proportional to the mass of the object. It is determined by the size of the elastic deformation. The spring force F=kx. The static friction force is equal to the external force causing the relative motion tendency. The sliding friction force f=μN is determined by μ and N.

2. Judgment of the existence of three kinds of nature forces

(1) All objects on the earth are affected by gravity.

(2) Elastic force is a passive force. There may not necessarily be an elastic force between objects in contact with each other.

① For cases with obvious deformation, we can directly judge from the conditions for the generation of elastic force. If the spring deforms, the spring will generate elastic force.

② For cases where the deformation is not obvious, the hypothesis method is usually used to judge.

(3) Friction is a passive force, and there may not necessarily be friction between objects in contact with each other.

① It is easy to judge whether sliding friction exists. We only need to make a judgment based on the conditions for generating friction.

②The difficulty in judging whether static friction exists lies in determining the relative motion trend, so the hypothesis method is often used to judge whether static friction exists.

As shown in the figure, a person stands on a long wooden board on the level ground and pushes the box to the right with force F. The wooden board, the person, and the box are all at rest. The masses of the three are all m, and the acceleration due to gravity is g, then ()

A. The direction of the friction force on the box is to the right

B． The direction of friction between the ground and the board is to the left

C. The pressure of the board on the ground is 3mg

D． If a person pushes the box diagonally downward, the pressure of the board on the ground will be greater than 3mg

As shown in the figure, logs are stacked on the horizontal ground. Regarding the direction of the force of log P at support points M and N, the following statement is correct ()

A. The supporting force at M is vertically upward

B． The support force at N is vertically upward

C. The static friction force at M is along the direction of MN

D． The static friction force at N is along the horizontal direction

Friction sudden change problem

When external factors or motion conditions change, the friction force on the object may change suddenly. The specific situations are classified as follows:

Force analysis of objects

1. Force analysis

Analyze all the external forces that the research object (specified object) receives in the specified physical environment, and draw a schematic diagram of the forces on the object. This process is force analysis.

2. Basis for force analysis

(1) Conditional judgment: judge whether the object is acted upon by a certain force based on whether the conditions for the generation of the force are met;

(2) Effect judgment: judge whether the object is acted upon by a certain force based on whether the effect of the force is reflected;

(3) Interaction judgment: Taking advantage of the reciprocity of force action, that is, the force-exerting object is also the force-receiving object. From whether one object is acted upon by a certain force, we can judge whether another object is acted upon by the corresponding force.

4. Steps of force analysis

(1) Clarify the research object: that is, determine the object for force analysis. The research object can be a single object or a combination of multiple objects.

(2) Analyze in a certain order: first gravity, then elastic force, friction force, finally analyze other forces (non-contact force), and draw a force diagram - draw the forces one by one on the force diagram while analyzing , accurately mark the direction of each force.

(3) Check whether each force drawn can find its force-exerting object, and check whether the analysis results can put the research object in the motion state given in the question. Otherwise, leakage, multiple forces or wrong forces must have occurred. ．

Summary and improvement

(1) When performing force analysis, we must pay attention to the fact that what we analyze is the force "received" by the object, not the force exerted by the object.

(2) Distinguish between internal forces and external forces: When performing force analysis on a whole composed of several objects, the forces acting between these objects are internal forces and cannot appear in the force diagram; when an object is analyzed in isolation, It turns out that the internal force has become an external force and must be drawn on the force analysis diagram.

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Update Time: 2024-06-24

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