The key points of mechanics in junior high school are sorted out, and the common force knowledge poi

Go buy this dragon gate, there are all of them.

Summary of knowledge points

Gravity, friction, and elasticity are our common forces, and they have different requirements for different forces

1. Gravity: It is necessary to grasp the force object of gravity, the magnitude, direction, application point of gravity and the influencing factors of gravity;

2. Friction: It is necessary to grasp the magnitude and direction of static friction, the direction of sliding friction, and the influencing factors of magnitude.

3. Elasticity: It is necessary to grasp the concept of elasticity. The magnitude and direction of the elastic force. Factors influencing the magnitude of elasticity.

Common test methods

Exam questions on this part. The examination basically appears in two ways, one is the multiple-choice questions that are not highly comprehensive, focusing on the examination of basic knowledge; The first is a fill-in-the-blank question with a high level of comprehensiveness. It mainly examines the degree of understanding of the above common forces.

Misunderstanding warning

1. The significance of friction in practice

1) "Favorable" friction: methods of increasing friction: increasing the roughness of the contact surface; increased pressure; Turn scrolling into sliding. (2) "Harmful" friction: ways to reduce friction: reduce the roughness of the rough surface; Reduced pressure; Change sliding to scrolling;

2. Conditions for the generation of sliding friction:

1) Rough contact surface;

2) two objects touching each other and squeezing each other;

3) There is relative motion between objects.

[Typical examples].

Example: Analysis:

This question examines whether the student can make a diagram of the force, and in addition, the question gives a disturbance condition, the ball moves upward, and some students think that the direction of the force on the ball should also be upward, thus ignoring that the direction of gravity is vertical downward.

Solution: The gravitational force on the ball is directed straight downward, and the force is 4 times the scale given.

The knowledge points of physics and mechanics in junior high school are as follows:1. The three elements of force: the magnitude, direction, and application point of force are called the three elements of force.

The method of using a line segment with an arrow to represent all three elements of force is called the force diagram method. The length of the line segment indicates the magnitude of the force; The direction of the arrow indicates the direction of the force; The starting point of the segment represents the point at which the force is applied.

2. Inertia: We call the property of an object that keeps its state of motion unchanged as inertia.

3. When several forces act together on an object, their effect can be replaced by a force, which is called the resultant force of those forces. If the magnitude and direction of several forces are known, finding the magnitude and direction of the resultant force is called the synthesis of the force.

4. Rolling friction: The friction generated when an object rolls on another object is called rolling friction.

5. Transforming sliding friction into rolling friction can greatly reduce friction.

The knowledge points of physics and mechanics in junior high school are summarized as follows:1. Elastic force or friction may occur between objects in contact with each other. On the other hand, the condition for the elastic force to occur is "contact and mutual extrusion to produce deformation".

2. There is also a strong effect between objects that are not in contact, such as the force of a magnet on iron, the effect of the earth on a bird flying in the air, etc., and we will learn about the force generated by this "field" in high school.

3. Concept: Force is the action of an object on an object.

4. Effect: force can change the motion state of the object; Force can change the shape of an object.

5. Unit: The unit of force in the International System of Units is Newton, referred to as the cow, which is represented by n.

6. The force experienced by an object near the ground due to the attraction of the earth is called gravity.