2. Centralized memory method: For example, the reflection law of light can be condensed into "three lines with equal angles", and the imaging law of flat mirror can be condensed into "the object and image are symmetrical, and the left and right are opposite".
3. Formula memorization method: For example, "An object has inertia, and its inertia property depends on its mass, regardless of motion or rest."
4. Comparative memory methods: such as inertia and inertia law, image and shadow, evaporation and boiling, pressure and pressure, series and parallel connection, etc. Compare the differences and connections and find out the similarities and differences.
5. Derive memory method: for example, deduce the calculation formula of liquid internal pressure. That is p=F/S=G/S=mg/s=pvg/s=pshg/=pgh.
6. Classified memory method: For example, the distance traveled per unit time is called speed, the work done per unit time is called power, the mass per unit volume of a substance is called density, and the pressure per unit area is called pressure. , can be classified as "unit ...................".
7. As the name implies, according to the names of buoyancy, tension and supporting force, it is easy to remember the direction of these forces.
8. Causality law (conditional memory method): If the conditions for using the left-handed rule are determined, the left-handed rule can be used according to the force generated by the current in the magnetic field; If the current moves in the magnetic field and generates current, please use the right-hand rule.
9. Chart memory method: you can use small cards, rotating cardboard, list boxes, etc. Classify and summarize the knowledge content into chart memory.
10, practical memory method: for example, making a dynamometer can help students remember the knowledge that the elongation of the spring is proportional to the external force.
Finishing the knowledge points of physics in Grade 8 1. Galileo's inclined plane experiment
1. The conclusion of the experiment is that under the same conditions, the more stable the plane is, the farther the car goes.
Galileo's inference is that in an ideal situation, if the surface is absolutely smooth, the object will always move at a constant speed.
3. The purpose of the three experiments is to ensure that the car starts to move along the plane at the same speed.
4. The excellence of Gaglio's inclined plane experiment lies not in the experiment itself, but in the unique method used in the experiment-idealized reasoning based on the experiment. It marks the real beginning of physics.
Second, Newton's first law.
1, background information:
(1) Galileo analyzed similar experiments and came to the conclusion that if the surface is absolutely smooth, the resistance of the object is zero, the speed will not slow down and it will always move at a constant speed.
(2) Descartes' supplement to Galileo's reasoning conclusion: If an object is not stressed, its direction of motion will not change.
(3) Newton, a British scientist, summed up the research results of Galileo and others, and summed up an important physical law: all objects always keep still or move in a straight line at a constant speed without force.
2. Contents:
When no force acts on all objects, they always remain stationary or move in a straight line at a uniform speed.
Key points of understanding: ① Newton's first law is further inferred and summarized on the basis of a large number of empirical facts, and has stood the test of practice, so it has become one of the basic laws of mechanics recognized by everyone. But there can't be no pressure around us, so it's impossible to prove Newton's first law directly through experiments.
Newton's first law tells us that an object can move in a straight line at a uniform speed without force, and an object can move in a straight line at a uniform speed without force, that is, force has nothing to do with the state of motion, so force is not the reason for producing or maintaining motion. Force is the reason to change the motion state of an object.
(3) "No force acts on it" has two situations: first, the object is not acted on by any force, which is an ideal situation; Second, the object is not affected by an external force in a certain direction. For example, if an object moves on a smooth horizontal plane and the friction can be ignored, then the object will not be affected by external forces on the horizontal plane.
(4) "Always keep" refers to "what was the past and what is the present", such as: it was static at first, and then it was static; At first it was moving, and then it kept moving in a straight line at the final speed.
Third, inertia.
1, definition: The property that an object keeps its state of motion unchanged is called inertia.
Key points of understanding: "keep the original state of motion" refers to the state when there is no force. That is, stationary or uniform linear motion.
2. Inertia is an attribute of an object. All objects have inertia under any circumstances, and the size of inertia is only related to the mass of the object, and has nothing to do with whether the object is stressed, whether it is stressed, whether it is moving or not, and the speed of movement.
Inertia is not a force. It's just a property of an object. Therefore, it cannot be understood as "subject to inertia".
Newton's first law is also called the law of inertia.
5, the difference between inertia and inertia law
Inertia is the property of an object under any circumstances, whether it is subjected to external force or not. The law of inertia is an objective law describing the motion of an object, provided that the object is not subjected to external forces. Inertia is closely related to the law of inertia. Because an object has inertia, it follows the law of motion pointed out by the law of inertia when it is not acted by external force. ① Inertia is an attribute of the object itself, and the law of inertia is the law of motion that the object follows when it is not stressed. (2) Any object has inertia under any circumstances, which is manifested as "hindering" the change of motion state; The law of inertia is conditional.
6, the three steps of inertia phenomenon explanation:
(1) clearly study which object and what kind of motion state it was in at that time;
(2) When an external force acts on a certain part of an object (or an external force acts on other objects associated with the object), the movement state of the part changes;
(3) The other part of the object still maintains its original motion state due to inertia;
(4) Finally, what happens.
7. Inertia in life:
People can't stop at once when they run to the finish line; After the emergency braking, the car can't stop immediately, and it has to drive forward for a certain distance.
8, the application of inertia:
① Tighten the loose hammer; 2 pat the clothes hard, you can pat off the dust on the clothes; (3) When the shovel is used to load soil into the car, the soil will be thrown to the car along the direction of shovel movement; (4) Sprinkle the water in the basin; ⑤ Long jump, you should run up first; ⑥ When fighting in ancient times, you can trip the enemy's galloping horses with a trip rope; ⑦ Turn off the engine in advance when the train enters the station; ⑧ The drying barrel of the washing machine can dry wet clothes when rotating at high speed; Pet-name ruby kick the football into the goal.
9, the harm of inertia and measures
Hazard: mainly some means of transportation, which is faster. When people brake and turn quickly, they have to keep the original motion state because of inertia, which is easy to cause accidents.
Measures: passengers in front of the minibus should wear seat belts; Install the airbag; Keep the distance between cars when driving; Speed limit; Packaging glass products should be padded with thick foam plastic.
Grade 8, 3, arrangement of physical knowledge points, 1, reference objects
(1), definition: An object assumed to be stationary for studying its motion is called a reference object.
(2) Any object can be used as a reference, and the choice of reference is usually based on the convenience of studying the problem. For example, when studying the motion of objects on the ground, the ground or objects fixed on the ground are often selected as reference objects, in which case the reference objects can be omitted.
(3) Choosing different reference objects to observe the same object may lead to different conclusions. Whether the same object is moving or static depends on the selected reference object, which is the relativity of motion and static.
(4), can't choose the research object itself as a reference, because the research object is always static.
Exercise (1) poem "Seeing the mountains meet like Buddha, but watching the mountains carefully is a boat trip", in which the reference objects of "seeing the mountains meet like Buddha" and "making a boat trip" are boats and mountains respectively.
(2) The passengers sitting in the eastbound A car saw the trees on the roadside retreating, and at the same time saw that the B car also retreated from the A car. Try to explain the action of the B car.
There are three situations: ① Car B does not move; ② Car B moves eastward, but the speed is not as fast as that of car A; ③ The B car is heading west.
(3) Explain Mao Zedong's poem "Sitting on the ground and traveling 80,000 miles a day, observing the sky and seeing the sea far away" in Seeing the Sorrow God.
The first sentence: Taking the center of the earth as a reference, the ground turns 80,000 miles around the center of the earth. The second sentence: Take the moon or other celestial bodies as a reference, and you can see many rivers on the earth here.
Second, the mechanical movement
1, definition: In physics, the change of object position is called mechanical motion.
2. Features: Mechanical motion is the most common phenomenon in the universe.
3. Methods of comparing the speed of objects: ① Compare the speed of pedestrians and cyclists who start at the same time: at the same time, the longer the distance, the faster the movement; ② Compare the speed of the 100-meter athletes: the shorter the time, the faster the movement for the same distance; ③ Compare the speed of 100-meter runners and 10,000-meter athletes. This method is often used to compare the speed of an object in practical problems and to describe the speed of motion in physics.
Exercise: In physical education class, students A, B and C run 100 meter. Their scores were 14.2s, 13.7s and 13.9s respectively, and the student won the first place. The easiest way to compare the speed of three races here is to run faster in the same time.
4. Classification: (according to the motion route) (1) Curve motion (2) Linear motion.
ⅰ uniform linear motion:
A. definition: the speed is constant, and the motion along a straight line is called uniform linear motion.
Definition: In uniform linear motion, the speed is equal to the distance traveled by the moving object in unit time. Physical meaning: Speed is a physical quantity representing the speed of an object.
Calculation formula: v t, t deformation s = v B, speed unit: in the international system of units, m/s unit is greater than km/h unit in m/s transportation.
Conversion: 1m/s = 3.6km/h People's walking speed is about 1. 1m/s, which means people walk at a constant speed.
Travel time is1.1m. From the image, we can see that the direct measuring tool: speedometer is moving at a constant speed. Velocity image: degree v is a constant and has nothing to do with travel time t.
Ⅱ variable speed movement:
A. definition: the movement with variable speed is called variable speed movement.
B, average speed: total time (to find the average speed of a certain distance, you must find the distance and the corresponding time).
C, physical meaning: indicates the average speed of variable speed movement.
D. Measurement of average speed: principle and method: measure the distance with a scale and measure the time with a stopwatch. A car that slows down on an incline. Set the upper half and the lower half, and the average speed of the whole journey is v 1, v2, and v is v2>v>v 1
E. Common sense: the walking speed of people is 1. 1m/s, the speed of bicycles is 5m/s, and the speed of jumbo jets is 900km/h..
8 vehicle speed 140 km/h high-speed vehicle speed 108km/h light speed and electric wave 3x10m/s/s.
Ⅲ. Recording of data in the experiment:
Designing data record forms is one of the basic abilities that junior high schools should have. When designing a table, it is necessary to find out the quantities directly measured and calculated in the experiment, and then find out the number of groups of data to be recorded as rows and columns of the table. You can design a reasonable table according to your needs.
practise
In a middle and long distance race, Xiaoming ran 1000m and Xiaohong ran 800m, which took 4 minutes 10 second and 3 minutes and 20 seconds respectively. Please design a record form and record their running distance, time and average speed in the form. Solution: The table design is as follows:
Third, the length measurement:
1, length measurement is the most basic measurement in physics and the basic skill of scientific inquiry. A common tool for measuring length is a scale.
2. In the international system of units, the main unit of length is m, and commonly used units are kilometer (km), decimeter (dm), centimeter (cm), millimeter (mm), micron (micron) and nanometer (nm).
3. Conversion relationship between main unit and common unit:
3369 1km = 10m 1m = 10dm 1dm = 10cm 1cm = 10mm 1mm = 10nm 3 1 μm = 10n m
The process of unit conversion: Formula: "Constant coefficient, equivalent substitution".
4. Length estimation: blackboard length is 2.5m, desk height is 0.7m, basketball diameter is 24cm, nail width 1cm, pencil lead diameter 1mm, new pencil length 1.75dm, palm width 1dm, and ink bottle height is 6cm.
5, special measurement method:
A> accumulation method is often used to measure the diameter of thin copper wire, the thickness of a piece of paper and other tiny quantities (when the measured length is small and the accuracy of the measuring tool is not enough, smaller objects can be accumulated, and then a single length can be obtained after measuring with a scale).
☆ How to measure the thickness of a piece of paper in physics textbooks?
Answer: Count the pages of physics textbooks, write down the total number of pages n, and measure the thickness L of n pieces of paper with millimeter scale, then the thickness of a piece of paper is L/N. ..
☆ How to measure the diameter of thin copper wire?
Answer: The thin copper wire is tightly wound around the pencil shaft for N turns to form a solenoid, and the length L of the solenoid is measured with a scale, so the diameter of the thin copper wire is L/N. ..
☆ Two coils of thin copper wire, one of which has a diameter of 0.3mm, and the label on the other has fallen off. If you are only given two identical new pencils, can you find out its diameter more accurately? Write the calculation process and mathematical expression of the diameter of thin copper wire. Answer: Two coils of thin copper wire with known diameter and unknown diameter are tightly wound on two identical new pencils, and the coil lengths are equal. Write down the number of winding turns N 1 and N2, and you can calculate the diameter D2 of the unknown copper wire = 0.3n1/n2mm.
B& gt； Measure the distance between two points on the map, the circumference of the cylinder, etc. , is often converted into a straight line method (by superimposing flexible wires that are not easy to stretch, marking the starting point and the ending point on the curve to be measured, and then straightening measurement).
☆ Given a soft copper wire and a scale, can you estimate the railway length from Beijing to Guangzhou with the atlas?
Answer: Concentrate the railway line from Beijing to Guangzhou with a thin copper wire lap diagram, then straighten the thin copper wire, measure the length L with a scale, find out the scale and calculate the length of the railway line.
C> Common wheel rolling methods, such as measuring the length of the playground runway (roll the wheel with known circumference along the curve to be measured, write down the number of turns of the wheel and calculate the length of the curve).
D> Common Auxiliary Methods for Measuring Coins, Spheres and Cylindrical Diameters Cones (For the length of objects that cannot be directly measured with the scale, it can be measured with the scale triangle).
☆ How many ways can you think of to measure the diameter of coins? (Introduction)
(1), ruler triangle auxiliary method. (2) Cut the edge of the folded coin with a circle of strokes, then fold it in half and measure the crease length. (3) Measure the circumference and find the diameter of the coin by rolling it on paper. ④ Place the coin flat on a ruler and read the length between the left and right tangent lines of the coin.
6, the use of the ruler rules:
A. [Selection]: Select the scale according to actual needs.
B. "Observation": Before using the scale, observe its zero scale line, measuring range and dividing value.
C, "put" when measuring the length with the scale, the scale should follow the measured straight line (close to the object, not skewed). Do not use worn zero score lines. (When measuring an object with a scale worn by the zero line, start from the whole scale)
D. "Look": When reading, the line of sight should be perpendicular to the ruler.
E. "Reading": In accurate measurement, it is necessary to estimate the next digit of the scale value.
F "Note": The measurement result consists of numbers and units. (It can also be expressed that the measurement result consists of accurate value, expected reading and unit).
Exercise: Two students measure the length of the same pen. The result of A is 12.82cm, and the result of B is 12.8cm. If there is no error in the measurement of two students, the reason for the difference is that the scale values of the two scales are different. If the scales used by these two students are both mm, then the result of student B is wrong. The reason is that there is no estimated value.
7. Error:
(1) Definition: The difference between the measured value and the real value is called error.
(2) Reason: The measuring tools measure human factors in the environment.
(3) Method to reduce the error: measure for many times and take the average value. With more complicated instruments
(4) Errors can only be reduced and can't be avoided, and errors caused by subjective carelessness can be avoided if the rules for using measuring instruments are not observed.
9. Relativity between motion and stillness: Whether the same object is moving or still depends on the selected reference object.
10. Uniform linear motion: constant speed and straight route. This is the simplest mechanical movement.
1 1. Speed: a physical quantity used to express the speed of an object.
12. Distance flown per unit time. Formula: s = vt v = s \t t = s \v v.
The unit of speed is: m/s; Kilometers per hour. 1 m/s =3.6 km/h
13. Variable speed movement: The moving speed of an object is variable.
14. Average speed: In variable-speed movement, the speed of an object within this distance can be obtained by dividing the total distance by the time spent, which is the average speed. Use the formula:; Daily speed refers to the average speed in most cases.
15. According to the available distance and time;
16. The timing tools invented by human beings are: sundial → hourglass → pendulum clock → quartz clock → atomic clock.
The eighth grade physics knowledge point combs 4 basic knowledge combs.
I. Measurement of length and time
2 unit of length: in the international system of units, the basic unit of length is meter (m), and other units are kilometer (km) and decimeter (dm).
Centimeter (cm), millimeter (mm), micron (micron), nanometer (nm)1μ m = 0.000001m; 1 nm = 0.000 0001meter. A common tool for measuring length: scale.
Second, the use of the scale:
(1) Pay attention to whether the zero scale line of the scale is worn, and the minimum scale value and range;
Third, the speed of movement.
1, the speed of the object's movement is expressed by speed.
At the same time, the longer the object travels, the faster the speed; The shorter the time it takes for an object to move the same distance, the faster the speed.
In uniform linear motion, the speed is equal to the distance traveled by a moving object in unit time.
In physics, in order to compare the speed of object movement, the method of "simultaneously comparing distances" is adopted, that is, the distance of object movement is divided by the time spent. In this way, when comparing the speeds of different moving objects, the time can be guaranteed to be the same. Calculation formula: v=S/t
Where: s-distance meter (m); T- time-second; Speed-m/s (m/s)
In the international system of units, the unit of speed is meters per second, and the symbol is m/s or m s- 1. The speed unit of kilometers per hour is commonly used in transportation, and the symbol is km/h or km h- 1, 1m/s = 3.6km/h. V=S/t, so the deformation can be obtained. Motion speed change
The changing movement is called variable-speed movement, and the speed of variable-speed movement is expressed by the average speed. Roughly, the formula of speed can also be used to calculate, and the average speed = total distance/total time. 3. Describe the speed of movement
Definition of average speed: describes the speed at which an object moves at variable speed within a certain distance (or within a certain time). Physical meaning: it reflects the speed of an object in the whole movement process.
Formula: v=s/t
Fourth, measure the average speed.
1. Use of stopwatch: Reading: The digital unit of the small circle in the table is min2. Measurement principle: the average speed calculation formula is v = s/t.
Acoustic phenomenon
First, the occurrence and spread of sound.
1 phenomenon, textbook P 13, figure 1. 1- 1 shows that all objects that make sounds are vibrating.
When you hold down the tuning fork, the pronunciation stops, which means the vibration stops and the sound stops. Sound source of vibrating object. 2. Sound needs medium to spread, and vacuum cannot spread sound.
Sound travels in the air as invisible sound waves. When the sound waves reach people's ears, the eardrum vibrates, and people hear the sound. 3. The speed of sound propagation in the medium is simply called the speed of sound.
Generally speaking, the sound of V is solid >: V liquid > V gas travels at a speed of 340m/s or 1224km/h in air at 15℃ and at a speed of 0m/s in vacuum. ..
Environmental protection angle noise refers to the sound that interferes with people's normal rest, study and work and the sound that people want to hear.
3. People use decibels (dB) to divide the sound level; The lower limit of hearing is 0 dB; In order to protect hearing, the noise should be controlled within 90dB；; In order to protect
Certificate of work and study, should control the noise is not more than 70dB；; In order to ensure rest and sleep, the noise should be controlled within 50dB.
4. Ways to reduce noise: at the sound source, in the process of propagation, at the human ear.
Verb (abbreviation for verb) Sound can be used to spread information and transmit energy.
Sort out the knowledge points of eighth grade physics 5. First, pay attention to the concept of physics.
Junior high school will learn many important physical concepts and laws, which is the basis for solving various problems. Therefore, to truly understand and master them, we should strive to achieve "five skills":
Will express: can remember and correctly describe the contents of concepts and laws.
Expressiveness: define the expression formula of concepts and laws and the scientific significance of each symbol in the formula.
It will be understood that the scope and conditions of use of the formula can be controlled.
Can deform: can accurately deform the formula and understand the meaning after deformation.
Can be applied: can apply concepts and formulas for simple judgment, reasoning and calculation.
Second, pay attention to drawing and reading.
In the physics course of junior high school, students will learn the diagram of force, simple mechanical diagram, circuit diagram and light path diagram. One kind belongs to painting, such as drawing a light path diagram. And try to draw with standard symbols, clear lines and rulers. The other belongs to pattern recognition, such as v-t images and s-t images of mechanical moving parts, and melting and solidification images of crystalline and amorphous parts with changing state of matter. Remember the most basic image, and make clear the physical meaning of each part in the image.