16 junior high school physics research methods! ! !

Observation is a method by which people investigate related things under natural conditions in order to understand the nature and laws of things. It is one of the common methods for people to collect, record and describe perceptual materials, and it is the most basic and direct research method. Simply put, observation is to look carefully. However, it is different from general observation. Observation is the perceptual activity of conscious organization under the guidance of the brain. So it is also called scientific observation.

Example: Boiling of water: Before using the thermometer, you should observe its range and recognize its scale value. During the experiment, we should pay attention to the two situations of bubbles rising in water before and during boiling, and the changes of thermometer pointer before and during boiling. When learning sound, students can observe the movement of small pieces of paper in the speaker, observe that the tuning fork is inserted into the water to stir the water, observe that the cricket knows that the sound is, and then find that the object making the sound is vibrating; In addition, there is the law of light reflection; Law of refraction of light; Convex lens imaging; What are the factors related to sliding friction?

2. Magnification method

Magnification method is the measurement of some small physical quantities often encountered in physical experiments. In order to improve the measurement accuracy, it is often necessary to adopt appropriate amplification methods and select corresponding measuring devices to amplify the measured data before measurement. The commonly used amplification methods are cumulative amplification, deformation amplification and optical amplification.

(1) Cumulative amplification method: The method of measuring the measured physical quantity in multiple times under the condition of simple superposition is called cumulative amplification method (superposition amplification method). Such as measuring the thickness of paper and the diameter of metal wire. , this method is often used in measurement; The advantage of cumulative amplification method is that the measured object is magnified several times before measurement without changing the measurement properties, thus increasing the effective digits of the measurement results and reducing the relative error of measurement. When using the cumulative amplification method, we should pay attention to two points: first, the measured value cannot change during the expansion process; Second, efforts should be made to avoid introducing new error factors in the expansion process.

(2) Deformation amplification method: deformation is the action of force, and the basic expression of deformation in mechanics is the change of volume, length and angle. The method of displaying deformation can be mechanical, electrical and optical, such as: displaying the change of volume through the change of liquid column length; Thermal expansion: lever amplification method shows.

(3) Optical amplification method: There are two commonly used optical amplification methods. One is to enlarge the visual angle of the measured object through optical devices to form an enlarged image, which is convenient for observation and identification, thus improving the measurement accuracy. Such as a magnifying glass, a microscope, a telescope, etc. The other is to indirectly amplify the micro-physical quantity to be measured by optical devices, and obtain the micro-physical quantity by measuring the amplified physical quantity. For example, the optical lever mirror ruler method for measuring small length and small angle change is a commonly used optical amplification method.

3. Control variable method

The control variable method refers to the research method of changing only one physical quantity by controlling several physical constants when discussing the relationship between multiple physical quantities, thus transforming it into multiple single physical quantities that affect a physical quantity. The reflection of this method in the experimental data table is that there is only one condition difference between the two experiments, and if the results of the two experiments are different, it is related to this condition. Otherwise it doesn't matter. On the contrary, if we want to study whether physical quantities are related to a certain factor, we should only make that factor different and other factors should be the same.

For example, when studying which factors are related to the resistance of a conductor, the control variable method is adopted for the convenience of research. In other words, we must choose two suitable wires at a time, measure their resistance, then compare them and finally draw a conclusion. In order to study the relationship between conductor resistance and conductor length, we should choose wires with the same material cross section. In order to study the relationship between conductor resistance and conductor material, we should choose wires with the same length and cross section. Study the factors that affect the effect; Study the influencing factors of liquid evaporation rate; Study the internal pressure of liquid; Study what factors are related to the potential energy of kinetic energy; Study the relationship between the tone of string sound and the thickness, tightness and length of string; Study the relationship between the heat absorbed by an object and the type, quality and temperature of the substance; Study the relationship between current and voltage resistance; Study which factors are related to electricity or electric heating; Study which factors are related to the force of charged conductor in magnetic field; This method is used to study the factors affecting the direction of induced current.

4. Simulation method

The so-called analogy is to "draw inferences from one instance" and "draw inferences from another instance". In fact, it is a kind of reasoning from special to special and from general to general. It is a logical thinking based on the similarity or similarity of two or two objects in some aspects, and they may be the same or similar in other aspects. It can help us understand more complicated experiments and difficult physical knowledge. Analogy is a method of reasoning. Different things can be described by analogy if they have the same or similar properties and mathematical forms. Analogy is an important way to put forward scientific hypotheses and make scientific predictions. Many hypotheses in the history of physics are established by analogy. Kepler once said, "We value analogical reasoning more than anything else".

Example: voltage and water pressure; Water flow and water flow; Internal energy and mechanical energy; Atomic structure and solar system; Water waves and electromagnetic waves; Letters and pigeons deliver letters; The formation of the concepts of strength and speed. The application of analogy in physics can guide students to acquire knowledge by themselves, help to make assumptions and speculate, help to ask questions and imagine the direction to solve problems. Analogy can stimulate students' intention to explore, guide students to explore, make students become conscious and active activities, and develop their thinking ability.

Analogy is a way of thinking that scientists often use. Although the conclusion drawn by this method is not necessarily reliable, it is logically creative. There are many examples of analogy, so we should pay more attention to it, summarize it constantly, and find more suitable examples for analogy.

5. Equivalent substitution method

The so-called equivalent substitution method is a kind of thinking method that transforms unfamiliar complex problems into familiar simple models for analysis and research on the premise of ensuring the same effect. It is widely used in physics.

Example: The concept of total resistance (equivalent resistance) is introduced when studying the relationship between series circuit and parallel circuit. Connecting several resistors in series circuit is equivalent to increasing the length of conductor, so the total resistance is greater than any series resistance, which is called equivalent resistance of series circuit. Connecting several resistors in parallel in a parallel circuit is equivalent to increasing the cross-sectional area of the conductor, so the total resistance is less than any parallel resistance, which is called the equivalent resistance of the parallel circuit. In circuit analysis, complex circuits that are difficult to analyze can be simplified into simpler equivalent circuits; When studying the relationship between two forces on the same line, the equivalent substitution method also uses the concept of resultant force.

6. Comparative method

Comparative method is a thinking process and method to determine the similarities and differences between research objects. Various physical phenomena and processes can be compared to determine their similarities and differences. Comparison is the premise of abstract generalization. By comparison, we can establish physical concepts and summarize physical laws. Comparison can also be used for identification and measurement. Therefore, comparative method is the most basic method often used in the study of physical phenomena. There are three types of comparative methods: ① seeking common ground while reserving differences. That is, two or more objects are compared to find their similarities. ② Comparison of seeking differences in similarities. That is, compare two or more objects and find out their differences. ③ Comprehensive comparison of similarities and differences. That is, comparing the similarities and differences between two or more objects.

For example, the engines of automobiles, ships, trains and airplanes are all different, but they are all devices that convert the internal energy released during fuel combustion into mechanical energy. Although both gasoline engines and diesel engines are internal combustion engines, they are different in structure, inhaled gas, ignition mode and application scope. Another example is the comparison between evaporation and boiling. The similarity between them is the vaporization process. The difference is that the temperature when the liquid occurs is different, the place where it occurs is different, and the phenomenon is different. The relationship between mass and volume can also be studied by comparative method; The relationship between gravity and mass; Gravity and pressure; Electricity and electricity, etc.

7. Transformation method

In physics, some invisible phenomena or physical quantities that are not easy to measure directly are usually recognized through some very intuitive phenomena or indirectly measured through physical quantities that are easy to measure. This method of studying problems is called transformation method. This method is used in the research and experiment of the concept law of junior middle school physics, and it is applied in many places.

For example, the deformation or change of motion state of an object can prove that some objects are stressed; The experiment of Madburg Hemisphere can prove the existence of atmospheric pressure. The appearance of fog can prove that the air contains water vapor; The formation of shadows can prove that light travels along a straight line; Eclipses can prove that the moon is not a light source; Oster experiment can prove that there is a magnetic field around the current; Compass refers to the north and south that can prove the existence of geomagnetic field; Diffusion phenomenon can prove that molecular motion is irregular; Lead block experiment can prove that there is gravity between molecules; If a moving object can do external work, it can prove that it has energy and so on.

8. Ideal experiment

Ideal experiment is also called "imagination experiment", "abstract experiment" or "thought experiment". It is an ideal process that people shape in their minds, a thinking process of logical reasoning and an important method of theoretical research. Although ideal experiment is called experiment, it is different from real scientific experiment in principle. The real scientific experiment is a practical activity, while the ideal experiment is a thinking activity. The former is an experiment that can be designed through physical process, and the latter is an experiment that abstract thinking can't achieve. However, the ideal experiment is not a subjective fantasy divorced from reality. First of all, the ideal experiment is based on practice. The so-called ideal experiment is to grasp the main contradiction and ignore the secondary contradiction on the basis of real scientific experiments, and make a deeper abstract analysis of the actual process. Secondly, the promotion process of ideal experiment is based on certain logical rules, summed up from long-term social practice and confirmed by practice.

Ideal experiment plays an important role in the ideal research of natural science. However, the ideal experimental method also has some limitations. Ideal experiment is only a logical reasoning thinking process, and its function is limited to logical proof and refutation, and it cannot be used as a standard to test correctness. On the contrary, any inference drawn from the ideal experiment must be tested by observing the experimental results.

Example: study whether vacuum can transmit sound; Newton's first law, etc.

9. Modeling method

Modeling method is a highly abstract physical model of ideal objects and forms, which can visualize abstract hypothesis theory and facilitate the imagination and thinking of research problems. The development process of physics can be said to be a process of constantly establishing physical models and replacing old or imperfect physical models with new ones.

Example: when studying the atomic structure that cannot be observed by naked eyes, establish the nuclear structure model; Light model is used to study light phenomena; Magnetic induction line model is used to study magnetic phenomena; The schematic diagram of force or the diagram of force is the model of actual object and force; Circuit diagram is the model of physical circuit; The principle and working process of generator are studied by wall chart and hand generator model. The structure and working principle of internal combustion engine are studied by using wall chart and diesel engine model of gasoline engine.

10. Balance table method

Balance is relative to the physical combination of more than two objects. In the process of physical changes, some physical quantities of objects in the combination remain unchanged under certain conditions. At this time, we call this state in which these objects are in equilibrium. The equilibrium problems in junior high school physics research can be summarized into the following three categories: first, the balance of objects under the action of balancing force; The second is lever balance: the third is the heat balance achieved by mixing objects with different temperatures. These three problems must be solved by the principle of balance.

Example: When you are playing with a wooden trolley model, let the hammer fall freely and pull the trolley forward. Among them, there is friction between the cart and the board, so the measured cart speed is wrong, so now we can balance the friction of the cart with a balanced method, such as raising the board.

1 1. Tracking method

In physical experiments, some physical phenomena are fleeting, and it is difficult for the experimenter to observe and measure the research object at this moment. Such as the position, trajectory and image of a moving object. But we can record the relevant information in a certain way, and then study it through measurement or observation. This method is the way to leave traces.

Example: the sand pendulum depicts the vibration curve of a simple pendulum; Record the position of the object with a point timer; Shoot the position of the flat ball with a strobe camera; Observe the waveform of AC signal with oscilloscope.

12. Cumulative method

It can not only conveniently measure some physical quantities that are difficult to be directly and accurately measured by conventional instruments, but also improve the measurement accuracy and reduce errors. This method is called accumulation method.

Master accumulation method: (1) Time accumulation method: the method of measuring the average after time accumulation. (2) Spatial accumulation method: the average method after accumulating space.

Example: in the experiment of "measuring gravity acceleration by dripping water", adjust and measure the height difference h from the faucet to the plate; Let the first drop of water fall on the plate, and when you hear the sound, the last drop of water just leaves the faucet; Then measure the total time tn of N times of water hammer disc sound, and it takes time to fall to the height of H. For example, in the experiment of "Measuring metal resistivity", if there is no screw micrometer, the metal wire can be wound around the pencil several times, and the diameter of the metal wire can be measured by dividing the total length of the metal coil by the number of turns.

13. Extrapolation method:

Some physical quantities can be observed or measured locally. Because of its extreme situation, it is not easy to observe intuitively. If the laws obtained by local observation and measurement are extrapolated to the extreme, the goal can be achieved. For example, in the experiment of measuring the electromotive force and internal resistance of the power supply, the terminal voltage (electromotive force) when I=0 (open circuit) and the current intensity when I = 0 (short circuit) cannot be directly measured. A straight line is drawn through a series of U and I corresponding value points and extends in two directions, with the U axis point as electromotive force and the I axis point as short circuit current.