Summary of physics knowledge points in the second volume of the eighth grade of the people's education edition
Chapter 7 Force
7.1 Force (F)
1. Definitions:
Note (1) The generation of a force must have a force object and a force object, and they exist at the same time.
(2) An object alone cannot produce a force.
(3) The action of force can occur between objects that are in contact with each other, or between objects that are not in direct contact.
2. The existence of judgment can be judged by the effect of force.
The force has two effects:
(1) (The change in motor status refers to and).
For example, if you push the trolley hard, the trolley changes from stationary to moving; The goalkeeper catches the flying football.
(2) Example: Pressing the spring hard, the spring is deformed; Pull the bow hard to deform the bow.
3. Units of force:
4. The three elements of force: , , , called. They all have an effect on the effect.
5. Representation of force: Draw a schematic diagram of force. Draw a line segment along the direction of the force on the stressed object, draw an arrow at the end of the line segment to indicate the direction of the force, the beginning or end point of the line segment represents the point of action of the force, and the length of the line segment indicates the magnitude of the force, this method is called the schematic diagram of the force.
7.2. Elasticity
(1) Elasticity:;
Plasticity:.
(2) Definition of elasticity: (e.g. pressure, support, pull)
(3) Generating conditions:
Spring dynamometer
(4) The tool for measuring the magnitude of the force is called a spring dynamometer.
How spring dynamometers (spring scales) work:. That is, the greater the tension experienced by the spring, the longer the elongation of the spring.
(5) Precautions for using spring dynamometer:
A. Observe the range and indexing value of the spring dynamometer, which cannot exceed it (otherwise the dynamometer will be damaged)
B. The pointer should be used before use; If it is not possible to adjust the zeroing, the number of the initial unmeasured should be subtracted after the reading to obtain the measured force.
C. Before measuring, gently pull the hook back and forth several times along the axis direction of the spring, and observe whether the pointer can return to the position of the original pointer after letting go, so as to check whether there is excessive friction between the pointer, the spring and the shell;
D. The direction of the measured force should be with the spring, so as to avoid excessive friction between the hook rod and the shell;
E. After the pointer is stable, the reading should be in line with the scale mark.
7.3 Gravity (G)
1 Cause: Due to the attraction between the earth and the object.
2 Definition: The force on which an object is subjected as a result; Indicated by letters.
3 Magnitude of gravity:
(1) Also known as (2) The gravitational force experienced by an object is proportional to its mass.
(3) Calculation formula: where g=,
Physical Significance:
4. Force-applied object:
5, Gravity Direction:,
Application: Reperpendicular line
(1) Principle: It is made by using properties.
(2) Function: Check whether the wall is vertical and whether the desktop is horizontal.
6. Point of action: (The center of gravity of an object with uniform mass distribution and regular shape is at its geometric center.) The center of gravity does not have to be on the object. )
7. For the convenience of studying the problem, when drawing the schematic diagram of the force on the stressed object, the action point of the force is often drawn on the center of gravity. When several forces are applied to the same object at the same time, the points of action are also drawn on the center of gravity.
Chapter 8 Movement and Force
8.1 Newton's First Law (also known as the Law of Inertia)
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: When all objects are in action.
3. Newton's first law is derived through experimental facts and scientific reasoning, and it cannot be directly verified by experiments.
4. Inertia
(1) Definition: The property that an object retains is called inertia
(2) Properties: Inertia is a property of an object. Everything is in.
(3) 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 the shape, velocity, whether the object is subjected to force and other factors.
(4) Prevent the phenomenon of inertia: the car is equipped with airbags, and the car is equipped with seat belts.
(5) The phenomenon of using inertia: long jump can improve performance, and patting clothes can remove dust.
(6) Explain the phenomenon:
For example, when a car brakes suddenly, why do passengers tip in the direction of the car?
Answer: Before the car brakes, the passenger is in a state of motion with the car, when braking, the passenger's foot suddenly stops with the car due to friction, and the passenger's upper body needs to maintain the original state of motion due to inertia, and continues to move in the direction of the car, so .......
8.2 Balance of two forces
1. Equilibrium state: , called equilibrium state.
2. Balance force: The force received is called balance force.
3. Two forces balance condition: the two forces acting on , if, , , (Equal-size, inverse, collinear, homogeneous)
4. Application of two-force equilibrium conditions:
(1) Judge the motion state of the object according to the force:
(1) When the object acts, the object is in a state of total preservation or equilibrium (equilibrium state).
(2) When the object acts, the total or state of the object (equilibrium state).
(3) When the object acts, the object's.
(2) Judge the force of the object according to the motion state of the object.
(2) When the object is in equilibrium (at rest or in a state of uniform linear motion), the object is not subject to force or is subject to a balanced force.
Note: When judging the equilibrium force on an object, it is important to first determine in what direction the object is in equilibrium (horizontal or vertical) before judging in what direction the object is in equilibrium.
(2) When the object is in a non-equilibrium state (accelerated or decelerated motion, change of direction), the object is subjected to the action of non-equilibrium force.
5. Conditions for the object to remain in equilibrium:
6. Force is the cause of changing the state of motion of an object, not the reason for maintaining the motion of an object.
8.3 Friction
1 Definition: When two objects occur, they produce a kind of force, which is called friction.
2. Generating conditions: A, object; B、。
3种类：A、B、静摩擦 C、
4 Factors affecting the magnitude of sliding friction: and .
5 Direction: Opposite to the direction of the object. (Friction is not necessarily resistance)
6. Method of measuring friction:
Principle:. (Balance of two forces)
7 ways to increase beneficial friction: A, B, .
8 ways to reduce harmful friction:
A、B．；
C、D、。
Chapter 9 Pressure
9.1. Pressure:
(1) Pressure
1. Definition: The force pressing on the surface of an object is called pressure. 2. Direction:
3. Point of action: 4. Size: Only when the object is naturally stationary in the horizontal plane, the pressure of the object on the horizontal support surface is in the same as the gravity of the object, there is: F=G=mg but the pressure is not gravity
(2) Pressure
1. The effect of pressure is related to and.
2. Physical significance: Pressure is a physical quantity represented.
3. Definition: The pressure on an object is called pressure.
4. Formula: P=F/S
5、单位： 1pa = 1N/m2
Meaning: Indicates that the object (ground, tabletop, etc.) is in.
6. Methods of increasing pressure: 1) Example:
2) Examples:
7. Methods to reduce pressure: 1) Examples:
2) Examples:
9.2. Liquid pressure
1. Cause: The liquid is subjected to action and has pressure on the bottom of the container that supports it;
The liquid has, and there is pressure on the side wall of the container.
2. Characteristics of liquid pressure:
1) The liquid has pressure on the container, and the liquid has pressure inside;
2) The pressure in all directions increases as it increases;
3) at the same depth, the pressure in all directions is;
4) At the same depth, the pressure of the liquid is also related to the density of the liquid, the larger the liquid, the greater the pressure.
3. Formula of liquid pressure:
Note: Liquid pressure is only related to and not to liquid. independent of the density of the object immersed in the liquid (depth is not height); When the shape of the solid is a cylinder, the pressure can also be extrapolated using this formula
When calculating the pressure of a liquid on a container, the pressure must first be calculated by the formula, and then the pressure must be obtained by the formula.
4. Communicator:
Features: When connecting the liquid in the container, the liquid depth in each container, that is, in each container.
Application examples:
9.3. Atmospheric pressure
1. The pressure generated by the atmosphere on the object immersed in it is called atmospheric pressure.
2. Cause: gas, and there is, so it can produce pressure on the object immersed in it in all directions.
3. Famous experiments to prove the existence of atmospheric pressure:
Other phenomena that prove the existence of atmospheric pressure: , .
4. The first experiment to accurately measure the atmospheric pressure value:
One standard atmosphere pressure is equal to the pressure generated, i.e. P0 = 1.013×105Pa, in a rough calculation, the standard atmospheric pressure can be taken, approximately supported.
5. The atmospheric pressure decreases with the increase of altitude, and within 3000 meters above sea level, the atmospheric pressure decreases with each increase; Atmospheric pressure is also affected by the climate.
6. Barometer and type:
7. Application examples of atmospheric pressure:
8. The boiling point of the liquid follows. (Application: Pressure cooker)
9.4. The relationship between fluid pressure and flow velocity
1. In physics, fluids and gases are collectively referred to as fluids.
2. In gases and liquids.
3. Application:
1) Passengers should stand outside the safety line while waiting for the bus;
2) The wing of the aircraft is made streamlined, and the speed of air flow on the upper surface is faster than that on the lower surface, so the pressure on the upper surface is small, and the pressure on the lower surface is strong, and there is a pressure difference between the upper and lower surfaces of the wing, so as to obtain upward lift;
Chapter 10 Buoyancy
10.1 Buoyancy (F float)
1. Definition: An object immersed in a liquid (or gas) will be subjected to an upward force, which is called buoyancy.
2. The direction of buoyancy is.
3. Cause: up and down from liquid (or gas) to the object.
4. Through experimental exploration, it is found (control variable method): the magnitude of buoyancy is related to and, the larger the volume of the object immersed in the liquid, the greater the density of the liquid, and the greater the buoyancy.
10.2 Archimedes' principle
1. Experiment: The relationship between the magnitude of buoyancy and the gravitational force expelled by the object to expel the liquid:
(1) Use a spring dynamometer to measure the G1 of the object and the gravity G2 of the keg;
(2) immerse the object in the liquid, read out the indication of the dynamometer at this time as F1, (calculate the buoyancy force F float = G1-F1 of the object) and collect the liquid discharged by the object;
(3) Measure the total gravity G3 of the liquid discharged by the keg and the object, and calculate the gravity of the liquid discharged by the object
with G row = G3-G2. Compare F float with G row
2. Contents:
The object immersed in the liquid experiences the upward buoyancy of the liquid, and the magnitude of the buoyant force is equal to the gravitational force expelled by the object to expel the liquid.
3. Formula: F float = G row = ρ liquid gV row
4. From Archimedes' principle, it can be seen that the magnitude of buoyancy is only determined by , and has nothing to do with the object's and being.
10.3 Float and sink conditions and applications of objects:
1. The floating and sinking conditions of the object:
| state | F floats with G objects | V row and V object | For solid objects ρ matter and ρ liquid |
| Kamibutsu | F floats > G objects | V row = V matter | ρ matter <ρ liquid |
| sink | F floats < G objects | ρ matter >ρ liquid | |
| levitation | F float = G matter | ρ matter = ρ liquid | |
| float | F float = G matter | V row < V object | ρ matter <ρ liquid |
2. Application of buoyancy
1) Ships use methods to increase buoyancy. Displacement of the steamer: . When a ship enters the sea from the river, the volume of the ship immersed in the water becomes smaller due to the greater density of the water, so it floats a little, but the buoyancy is not the same (always equal to the gravitational force on the ship).
2) Submarines are relied on to achieve ascent or diving.
3) Balloons and airships are leaning to change buoyancy.
4) The density meter works on the liquid level, and its scale is.
4. Calculation of buoyancy:
Pressure difference method: F float = F up - F down
Weighing method: F float = G matter - F pull (this method is generally used when the spring dynamometer condition appears in the question)
Floating and levitation method: F float = G matter (two forces balance)
Archimedes' method: F float = G row = ρ liquid gV row (this method is generally used when the volume condition appears in the problem)
Chapter 11 Work and Mechanical Energy
Section 1 Work
1. The preliminary concept of work: If a force acts on an object, and the object moves a certain distance in the direction of this force, it is said that the force has done work.
2. Work consists of two necessary factors: one is the force acting on the object, and the other is the distance the object moves in the direction of this force.
3. Calculation of work: work is equal to the product of the force and the distance that the object passes in the direction of the force (work = distance in the direction of force × force).
4. The formula for calculating work: W=Fs
F is used to represent force, the unit is ox (N), s is used to represent distance, the unit is meter (m), the symbol of work is W, the unit is bull • meter, it has a special name called joule, and the symbol of joule is J, 1 J = 1 N•m.
5. When vertically lifting an object to overcome gravity and do work, the calculation formula can be written as W=Gh; When doing work over friction, the calculation formula can be written as W=fs.
6. The principle of work; When using machinery, people will not do less work than when they do not use machinery (but directly by hand), that is to say, no work is saved when using any machinery.
6. When factors such as friction and the weight of the machine itself are not considered, the work done by people using machinery (Fs) is equal to the work done directly by hand (Gh), which is an ideal situation and the simplest situation.
Section 2 Power
1. The physical meaning of power:
2. Definition of power:
where W stands for work, and the unit is coke (J); t stands for time in seconds (s); F stands for tensile force in ox (s); v stands for velocity, and the unit is m/s; P stands for power, the unit is watts, abbreviated as watts, and the symbol is W.
4、功率的单位是瓦特（简称瓦，符号W）、千瓦（kW）1W=1J/s、1kW=103W。
Section 3 Kinetic energy and potential energy
First, the concept of energy
If an object is able to do work externally, we say it has energy. The units of energy and work are both joules. An object with energy is not necessarily doing work, and an object doing work must have energy.
2. Kinetic energy
1. Definition: The energy possessed by an object due to motion is called kinetic energy.
2. The factors that affect the magnitude of kinetic energy are: the mass of the object and the speed of the object's motion. For an object of the same mass, the greater the speed of motion, the greater its kinetic energy; For an object moving at the same velocity, the greater the mass, the greater its kinetic energy.
3. All moving objects have kinetic energy, the kinetic energy of stationary objects is zero, and the kinetic energy of objects moving at a uniform speed and with a certain mass (regardless of uniform speed rise, uniform speed decrease, uniform speed forward, uniform speed backward, as long as it is a constant speed) remains unchanged. The sign of whether an object has kinetic energy or not is: whether it is moving or not.
Second, potential energy
1. Potential energy includes gravitational potential energy and elastic potential energy.
2. Gravitational potential energy:
(1) Definition: The energy determined by the height of an object is called gravitational potential energy.
(2) The factors affecting the gravitational potential energy are: the mass of the object and the height of the lifted. The higher an object of the same mass is lifted, the greater the gravitational potential energy; The greater the mass of an object being lifted at the same height, the greater the gravitational potential energy.
(3) It is generally believed that the gravitational potential energy of an object on a horizontal ground is zero. The gravitational potential energy of an object with an increased position and a certain mass (whether it rises at a uniform velocity, accelerates, or decelerates, as long as it is raised) increases, and the gravitational potential energy of an object with a reduced position and a certain mass decreases (whether it decreases at a uniform velocity, acceleration, or deceleration, as long as it decreases), and the gravitational potential energy of an object with a constant height and a certain mass does not change.
3. Elastic potential energy:
(1) Definition: The energy possessed by an object due to elastic deformation is called elastic potential energy.
(2) The factors affecting the magnitude of elastic potential energy are: the magnitude of elastic deformation (for the same elastic object).
(3) For the same spring or the same rubber band, the greater the deformation (within a certain elastic range), the greater the elastic potential energy. A sign of whether an object has elastic potential energy or not: whether elastic deformation occurs or not.
Section 4 Mechanical energy and its transformation
1. Mechanical energy: kinetic energy and potential energy are collectively referred to as mechanical energy. Kinetic energy is the energy that an object has when it is in motion, and potential energy is the energy that is stored. Kinetic energy and potential energy can be converted into each other. If only kinetic energy and potential energy are converted into each other, the sum of mechanical energy remains unchanged, i.e., mechanical energy is conserved.
2. The law of transformation between kinetic energy and gravitational potential energy:
(1) For an object with a certain mass, if it accelerates and falls, the kinetic energy increases, the gravitational potential energy decreases, and the gravitational potential energy is converted into kinetic energy;
(2) For an object with a certain mass, if it decelerates and rises, the kinetic energy decreases, the gravitational potential energy increases, and the kinetic energy is converted into gravitational potential energy.
3. The transformation law between kinetic energy and elastic potential energy:
(1) If the kinetic energy of one object decreases and the elastic potential energy of another object increases, the kinetic energy is converted into elastic potential energy;
(2) If the kinetic energy of one object increases and the elastic potential energy of another object decreases, then the elastic potential energy is converted into kinetic energy.
4. The mechanical energy available to human beings in nature includes water energy and wind energy. Large hydropower stations raise the water level by building barrages, thereby increasing the gravitational potential energy of the water in order to convert more mechanical energy into electrical energy when generating electricity.
Chapter 12 Simple Machinery
Section 1 Leverage
1. Definition: A hard rod, if it can rotate around a fixed point under the action of force, this hard rod is called a lever.
2. Five elements: one point, two forces, and two force arms. ((1) The "point" is the fulcrum, the point around which the lever turns, denoted by "O". (2) The "two forces" are power and resistance, and their points of action are on the lever. Power is the force that makes the lever rotate, generally represented by "F1", and resistance is the force that hinders the lever from turning, generally represented by "F2". (3) "Two force arms" are power arms and resistance arms, the power arm is the distance from the fulcrum to the power line, which is generally represented by "L1", and the resistance arm is the distance from the fulcrum to the resistance line, which is generally expressed by "L2". ）
3. The balance of the lever (the lever does not rotate or rotates at a constant speed under the action of power and resistance is called lever balance) The conditions are:
Power × power arm = resistance × resistance arm;
公式：F1L1＝F2L2。
4. Application of leverage
(1) Labor-saving levers: L1>L2, F1<F2 (labor-saving and distance-saving, such as: crowbars, guillotines, movable pulleys, axles, claw hammers, wire cutters, trolleys, flower branch scissors. ）
(2) Laborious levers: L1<L2, F1>F2 (laborious distance-saving, such as: human forearms, hair scissors, fishing rods. ）
(3) Equal arm lever: L1 = L2, F1 = F2 (no effort, no distance, can change the direction of the force Specific application of equal arm lever: balance. Many scales that weigh mass, such as rod scales and portable scales, are made according to the lever principle. ）
Section 2 Pulleys
1. The pulley is a deformed lever.
2. Fixed pulley:
(1) Definition: A pulley in which the shaft in the middle is fixed and does not move.
(2) Substance: Equal arm lever.
(3) Features: The use of fixed pulley can not save effort, but can change the direction of power.
(4) For the ideal fixed pulley (excluding the friction between the axles) F=G object. Rope free end movement distance SF (or velocity vF) = distance of the weight moving SG (or velocity vG)
3. Movable pulley:
(1) Definition: A pulley that moves with a heavy object. (Can be moved up and down, left and right)
(2) Substance: The power arm is a labor-saving lever twice that of the resistance arm.
(3) Features: The use of movable pulleys can save half of the force, but can not change the direction of power.
4. a pulley block
(1) Definition: fixed pulley and movable pulley are combined into a pulley group.
(2) Features: The use of pulley block can not only save effort but also change the direction of power.
Section 3 Mechanical Efficiency
1. Useful work: Definition: Useful work for people.
Formula: W useful = Gh (lifting weight) = W total - W amount = ηW total
Bevel: W useful = Gh
2. Extra work: Definition: It is not the work that we need but have to do.
Formula: W amount = W total - W useful = G moving h (movable pulleys and pulley sets that ignore axle friction)
Slope: W = fL
3. Total work: Definition: the work done by using work plus additional work or power
斜面：W总＝fL+Gh＝FL
4. Mechanical Efficiency: Definition:
5、。 It is usually expressed as a percentage. The mechanical efficiency of a pulley is 60%, which means that the useful work accounts for 60% of the total work.
6. Methods to improve mechanical efficiency:
7. Measurement of mechanical efficiency:
(2) The physical quantity that should be measured: the gravity of the hook code G, the height h of the hook code lifting, the tensile force F, and the distance S of the free end of the rope.
(3) Equipment: In addition to hook codes, iron frames, pulleys, and thin wires, scale rulers and spring dynamometers are also required.
(4) Steps: The spring dynamometer must be pulled at a uniform speed to raise the hook code, and the purpose is to ensure that the size of the dynamometer remains unchanged.
(5) Conclusion: The main factors affecting the mechanical efficiency of pulley block are:
(1) The heavier the movable pulley, the more the number of additional pulleys.
(2) The heavier the lifting weight, the more useful work will be done.
(3) Friction, if the friction is greater, the extra work will be done.
8. The winding method and the lifting height of the heavy object do not affect the mechanical efficiency of the pulley.