Crazy for all the knowledge points of force and movement in Physics Volume II of Grade 8

Crazy for all the knowledge points of force and movement in Physics Volume II of Grade 8

1. The linear motion of an object (or a particle) under the action of constant force is the uniform variable speed linear motion. 2. The typical uniform variable speed linear motion includes: (1) the free falling and vertical throwing motion of an object only under the action of gravity; (2) the motion of charged particles in a uniform electric field

Third year physics movement and force knowledge summary, force and mechanical knowledge summary, thank you!
People's education press, thank you!

1. Definition of force
Definition: force is the action of an object on an object
Explanation: the "function" in the definition is an abstract summary of the specific actions of pushing, pulling, lifting, lifting and pressing
2. Understanding of the concept of force
When a force occurs, there must be two (or more) objects. That is to say, without objects, there will be no powerful effect (materiality of force)
When a body is under the action of a force, there must be another body exerting a force on it. The body under the force is called the body under the force, and the body under the force is called the body under the force. Therefore, there is no force without the body under the force or without the body under the force
There is no force between the objects that contact each other, and there is no force between the objects that do not contact each other
The interaction of forces between objects is mutual
The interaction between the object exerting force and the object under force is mutual
The object applying force and the object under force are relative. When the research object changes, the object applying force and the object under force also change
3. The effect of force -- from this we can judge whether there is force
(1) It can change the movement state of the object
Note: the change of movement state includes the change of movement speed or direction
(2) It can change the shape and size of an object
4. Unit of force
In the international system of units, the unit of force is Newton, abbreviated as cow, represented by the symbol n
The size of 1n is equivalent to the force to pick up two eggs
5. Force measurement
Tools: dynamometer, the commonly used dynamometer in the laboratory is the spring scale
Principle of spring scale: the greater the tension on the spring, the longer the extension of the spring
6. Correct use of spring scale
Observe whether the range, division value and pointer of the spring scale point to the zero line
When reading, the line of sight, pointer and scale mark should be at the same level
The direction of the measured force should be consistent with the direction of the spring extension
7. Three elements of force
The size, direction and action point of force are the three elements of force, which can influence the effect of force
8. Diagram of force: the three elements of force are represented by a line segment with an arrow
9. Drawing method of force diagram
(1) Draw a stressed object: it can be represented by a square or rectangle, and a sphere can be represented by a circle
(2) Determine the action point: the action point is drawn on the stressed object and at the midpoint of the contact surface between the stressed object and the applied object. If the stressed object and the applied object do not contact or the same object is subjected to more than two forces, the action point is drawn at the geometric center of the stressed object
(3) Determine the scale: for example, how many Newtons are represented by 1 cm line length
(4) Draw line segment: draw a straight line along the direction of the force according to the scale from the action point of the force, which is used to represent the force
(5) Mark the direction of the force: draw an arrow at the end of the line segment (included in the line segment), indicating the direction of the force
(6) Mark the symbol and value of the force shown near the arrow
10. Schematic diagram of force
In some cases, we only need to describe the force of the object qualitatively, and do not need to express the force accurately
11. The concept of gravity
Definition: the force exerted by objects near the ground due to the attraction of the earth is called gravity (symbol: G)
Theory: 1. The earth is the force exerting object of gravity, and its force bearing object is all objects near the ground. 2. The magnitude of gravity is related to the mass of the object
12. Three elements of gravity
Size: g = mg
Direction: always vertical down (vertical horizontal facing down)
Point of action: the point of action of gravity is on the center of gravity of the object. The center of gravity of the object is in its geometric center with regular shape and uniform mass distribution
13. Types of friction
Sliding friction, rolling friction, static friction, rolling friction force is far less than sliding friction
14. Influence factors of sliding friction
① It is related to the pressure between objects and the roughness of contact surface
It has nothing to do with the speed of the object and the size of the contact surface
15. Methods of increasing beneficial friction and reducing harmful friction
Increase beneficial friction: ① increase the pressure between objects; ② increase the roughness of contact surface
Reduce harmful friction: ① reduce the pressure between objects; ② reduce the roughness of contact surface
16. The concept of resultant force
Resultant force: if the effect of a force is the same as that of two forces, the force is called the resultant force of the two forces
Theory 1: the concept of resultant force is based on "equivalence", that is, "resultant force" replaces component force, so resultant force is not another force acting on the object, it just replaces the original two forces. Don't mistakenly think that the object is also affected by resultant force. 2. The condition of the combination of two forces is that the two forces must act on one object at the same time, otherwise it is meaningless to seek resultant force
17. Synthesis of forces
Given the magnitude and direction of several forces, finding the magnitude and direction of the resultant force is called the composition of forces
(1) When two forces have the same direction, the resultant force is equal to the sum of the two forces; the direction is the same as the direction of the two forces
(2) When the directions of the two forces are opposite, the resultant force is equal to the difference between the two forces, and the direction is the direction of the greater force
9、 Force and motion
1. Balance force
Balance force: when an object can keep still or move in a straight line at a constant speed under the action of two forces, the two forces are called a pair of balance forces, or two force balance
The conditions (or characteristics) of equilibrium force: isomorphic, equivalent, reverse, collinear
Whether the two forces act on the same body is the key of whether the two forces are a pair of equilibrium forces or a pair of interaction forces
2. Newton's first law
Content: all objects in the absence of external force, always keep static or uniform linear motion
Theory (1) it contains two meanings: (1) a stationary object always remains stationary when it is not subjected to external forces
② A moving object always keeps a uniform linear motion when it is not affected by external force
(2) Newton's first law is the law of ideal
(3) An object must be in a state of static or uniform linear motion without force, but an object in a state of static or uniform linear motion may not be without force
3. Inertia
Inertia: the property that an object keeps its original state of motion unchanged is called inertia
Inertia is the inherent property of an object. All objects have inertia in any case
② The size of inertia is only related to the mass of the object, but has nothing to do with whether the object moves, how fast it moves or whether it is subjected to external forces
③ Note: inertia is not "force". In narration, don't say "the object is under the action of inertia" or "under the action of inertia"
[memory method]
(1) The doggerel of inertia understanding "objects have inertia, and the attributes of inertial objects depend on the mass, no matter whether they are dynamic or static"
(2) Understanding of the relationship between force and motion
No force
The force analysis shows that the resultant force is 0 and the state remains unchanged
Force balance
The state changes when the resultant force of non-equilibrium force is not zero
No force at rest
Constant equilibrium force in uniform linear motion
State analysis motion linear motion variable speed linear motion
The curve movement state changes the non-equilibrium force

Second year physics people's Education Press
Chapter 9: electricity and magnetism section 6
I don't know the power on guide line, stress direction and balance position
There is also the difference between the motor and the generator

Second year physics grade 8 Volume 2 (PEP)
[video introduction]
There are 86 lectures in Volume 2 of physics of Grade 8 of junior high school (PEP). EDQ parent child resource network for special grade physics teachers in middle school
Second grade physics, eighth grade physics tutor edition Speaker: Peng Menghua EDQ parent child Resources Network
Peng Menghua, a senior physics teacher in middle school, is now the executive director of Beijing Physics Society, vice president of Xicheng District Education Society of Beijing, and master's supervisor of comprehensive education in Physics Department of Beijing Normal University. He has won the titles of "Beijing Science and technology gardener" and "Beijing young and middle-aged backbone teacher"

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53°40′30″×2-72°57′28″÷2
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53°40′30″×2=107°21′
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70 ° 52 ′ 16 ″

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