What are the key points of knowledge in 8th grade physics?

1. The concept of force: Force is the action of an object on an object.

2. Conditions for force generation: There must be two objects. There must be interaction between objects (no contact is possible).

3. The nature of the force: the action of the force between objects is mutual (the interaction force is equal in magnitude and opposite in direction in any case, acting on different objects). When two objects interact, the object of force is also the object of force, and vice versa, the object of force is also the object of force.

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

Note: Whether the motion state of the object changes generally refers to whether the speed of the object changes (the change of speed) and whether the direction of motion of the object changes. When an object is deformed or its state of motion changes, it can be judged that it has been subjected to a force.

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

Friction (f).

1) Definition: When two objects in contact with each other are about to move or have been in relative motion, a force that hinders the relative motion will be generated on the contact surface, which is called friction.

2) Classification: Friction is divided into static friction and dynamic friction, of which dynamic friction is divided into sliding friction and rolling friction.

3) F slip = n.

where f slip: sliding friction; the coefficient of friction, which is related to the roughness of the object itself; n: Pressure (solid on a horizontal plane, pressure = gravity).

4) The magnitude of rolling friction is also related to the roughness of the object and the magnitude of the pressure; The magnitude of the static friction force is equal to the magnitude of the external force on the same straight line.

1. The influence of resistance on the motion of the object: let the same trolley slide freely from the same height of the same inclined plane (control variable method), in order to make the trolley slide to the bottom of the inclined plane with the same speed; The amount of resistance is expressed by the length of the distance the car slides on the board (conversion method).

2. The content of Newton's first law: all objects always remain at rest or in a state of uniform linear motion when they are not subjected to force.

3. Newton's first law is derived through experimental facts and scientific reasoning, and it cannot be directly verified by experiments.

4. Definition of inertia: The characteristic of an object that maintains its original state of motion unchanged is called inertia. Nature:

Inertia is a property inherent in the object itself. All objects have inertia at any time and in any state. Inertia is not a force, it cannot be said that inertia acts, the magnitude of inertia is only related to the mass of the object, and has nothing to do with factors such as the shape, velocity, and whether the object is subjected to force.

5. There are two necessary factors of work: one is the force acting on the object; The second is the distance the object passes in the direction of the force.

6. Calculation of work: work (w) is equal to the product of force (f) and the distance (s) of the object in the direction of force. (work = force distance).

1.If an object is subjected to a force and there is a displacement in the direction of the force, we are said to have done work on the object.

2.The formula for work: w=fs.

3.Two factors that do work:

1) The force acting on the object.

2) The distance the object travels in the direction of this force.

4.Compare the speed of the work.

Method 1: Do the same work, than time. The shorter the time, the faster the work will be done.

Method 2: The time is the same, and the work is done. The more work you do, the faster you do it.

Method 3: The work done and the time are not the same, and the ratio is different.

The greater the value of the work time, the faster the work is done.

1.Mechanical energy is the sum of kinetic energy and potential energy, and here potential energy is divided into gravitational potential energy and elastic potential energy.

2.It is mass and velocity that determine kinetic energy; It is the mass and height that determine the gravitational potential energy; The elastic potential energy is determined by the stiffness coefficient and the deformation.

3.Kinetic energy: The energy that an object has due to its motion, known as the kinetic energy of the object.

4.Relationship between potential energy and kinetic energy: The amount of increase in kinetic energy is equal to the amount of decrease in gravitational potential energy.

1. Definition: A hard rod that can rotate around a fixed point under the action of force is a lever.

2. Lever balance conditions: power power arm = resistance resistance arm, that is:

3. Application of leverage:

1) Labor-saving lever: the lever of the power arm is larger than the resistance arm, which saves effort but consumes distance.

2) Laborious lever: The lever of the power arm is smaller than that of the resistance arm, which is laborious but saves distance.

3) Equal arm lever: The power arm is equal to the lever of the resistance arm, which is neither labor-saving nor labor-intensive.

1.Frictional force.

When two objects in contact with each other are about to move or have been in relative motion, a force that hinders the relative motion is generated on the contact surface, which is called frictional force.

2.The conditions under which friction is generated.

1) The two objects are in contact and squeezed.

2) The contact surface is rough.

3) Relative motion is about to occur or has already occurred.

3.Classification of frictional force.

1) Static friction: The friction that will occur when relative motion is about to occur is called static friction.

2) Sliding friction: Relative motion belongs to sliding, and the friction generated is called sliding friction.

3) Rolling friction: The relative motion belongs to rolling, and the friction generated is called rolling friction.

4.Sliding friction.

1) Determinants: the amount of pressure between objects, the roughness.

2) Direction: Opposite to the relative direction of motion.