1 the relationship between conjecture and hypothesis
Conjecture and hypothesis are hypothetical explanations of the causality and regularity of things in the problem. As a form of rational thinking, conjecture and hypothesis are important methods in scientific research. Judging from its formation, it can be divided into two links: one is conjecture and the other is hypothesis. Conjecture is a variety of assumptions made by students on the basis of existing knowledge and experience, combined with their own perceptual knowledge of objective reality, and relying on intuition after contacting problems. Hypothesis is a more scientific hypothesis based on conjecture, which is obtained by eliminating some impossible conjectures through a series of logical reasoning such as observation, experiment, analysis, comparison and induction. Hypothesis is more reasonable than conjecture, and discussing problems is more targeted and instructive. In the guessing stage, students give full play to their subjectivity and actively put forward as many guesses and possibilities as possible, regardless of the causal logical relationship between questions and guesses, so their thinking is often in a very active, illogical and divergent state. In the hypothetical link, through the investigation of conjecture, we need a kind of logical aggregation thinking to make various explanations. In scientific inquiry, putting forward reasonable guesses and assumptions can not only point out the direction for inquiry activities, but also fully develop students' creative thinking and cultivate their innovative ability.
2 conjecture and hypothesis principle
2. 1 Rationality principle
Guess is not a shot in the dark. Unreasonable guesses and assumptions are not only impossible to develop into scientific conclusions, but also meaningless to the inquiry process. In the process of solving problems, we should not rely on blind attempts, but should put forward conjectures and assumptions to solve problems according to existing theories, our own experience, relevant materials collected, facts and the unique imagination and creativity of human beings. For example, when exploring the factors affecting evaporation rate, students put forward the hypothesis that evaporation rate is related to liquid surface area, air velocity above liquid and liquid temperature according to their experience of drying clothes in ventilated and sunny places, which has certain factual basis and is reasonable.
2.2 Principle of regularity
Although conjecture and hypothesis are put forward for some objective facts and phenomena, the purpose of scientific inquiry is not to solve individual problems, but to find the general relationship and laws between things as a kind of problems. Therefore, the conjectures and assumptions put forward should have certain regularity. For example, when discussing the reasons for the formation of light spots in the shade, if it is put forward that "circular light spots are formed by the circular sun, which has nothing to do with the shape of small holes", there is no regularity. What kind of spot will light of other shapes form after passing through the small hole? In this study, it is proposed that "the light emitted by the light source will form a light spot with the same shape as the light source on the screen after passing through the small hole on the barrier, regardless of the shape of the small hole". The phenomenon of the sun as a light source is extended to the law that all light sources follow, and the pinhole imaging law is obtained after hypothesis verification.
2.3 Directional principle
Conjecture and hypothesis is a kind of scientific foresight activity to study the problem before inquiry, which guides the formulation of inquiry plan and the design of scheme, and is the basis for students to determine the research direction and choose experimental methods and equipment. Therefore, the conjectures and assumptions put forward should make students clear the direction of inquiry and guide the whole inquiry activity. For example, in the above example, after making assumptions, students will choose different shapes of light sources, such as fluorescent lamps, light bulbs and candles, and drill holes with different shapes (such as circles, squares, triangles, etc.). ) on the opaque barrier, and then observe the spot shapes of light sources with different shapes after passing through different holes.
2.4 the principle of openness
Conjecture and hypothesis are the most active stages of middle school students' thinking in scientific inquiry. Different students have different understandings of the problem because of their different experiences, knowledge and abilities, so they will put forward different assumptions. In order to arouse the enthusiasm of all students and cultivate the ability of independent thinking and innovation, inquiry teaching requires each student to put forward his own assumptions to solve problems, and teachers should not interfere with students too much to ensure the openness of conjectures and assumptions put forward. For example, when exploring the phenomenon that the lights at home suddenly go out at night, students will put forward various assumptions, such as "power failure, fuse broken, filament broken, circuit problem" and so on. Teachers should "pretend to be deaf and dumb" when students make assumptions, so that students can make assumptions as much as possible and realize the openness of inquiry teaching.
3 strategies to cultivate students' guessing
3. 1 Create informative question situations to let students know what to guess.
Inquiry teaching attaches great importance to the creation of problem situations, thus stimulating students' curiosity and thirst for knowledge, stimulating students' interest in learning and desire to explore, and allowing students to find and ask questions. Because students can't make scientific guesses and assumptions according to their rich knowledge, experience and keen sense like scientists when facing complex natural phenomena, teachers should highlight some information about problem-solving methods or answers on the basis of objective facts when creating problem situations, so that the created situations can have a certain suggestive effect on students' guesses and assumptions. In this way, students will have a clear direction when making guesses and assumptions, so as not to make endless guesses and assumptions, and at the same time cultivate students' ability to collect information. For example, when exploring the concept of stress, provide a scene: a student is playing by the river and sees two ladies walking by the river. One wears high heels and the other wears flat shoes. Although they look the same in weight, there are obvious differences in the depth of footprints left on the wetland by the river. The imprint on the heel of high-heeled shoes is narrow and deep, while the imprint on flat-heeled shoes is wide and shallow. The student thinks there may be some truth in this. After students ask the question about which factors are related to the effect of pressure, according to the information in the topics "One wears high heels and the other wears flat shoes" and "The weight is equivalent", it is judged that the effect of pressure is related to the stress area and pressure. "The impression on the heel of high-heeled shoes is narrow and deep, while the impression on flat-heeled shoes is wide and shallow" shows that the effect of pressure is inversely proportional to the stress area, thus putting forward the conjecture of the answer to the question.
3.2 Teaching students the method of guessing
(l) Use experience and intuition to guess
Students have formed a lot of daily experience and knowledge in their daily life and study, which is the direct source and material for students to guess. Intuitive thinking is a kind of thinking that makes a reasonable guess, assumption or sudden understanding of the answer to a question quickly without step-by-step analysis. It often forms a spark of wisdom, which is inspired by creation. Making full use of students' experience and intuition in inquiry teaching is an effective means to cultivate students' guessing ability. For example, in the inquiry teaching of Newton's first law, teachers and students do slope experiments together and see three different situations on the surface: the smoother the slope, the farther the car moves. Students intuitively feel that if the slope is very gentle, the car will move far. Therefore, a conjecture is put forward: a moving object will always move in a straight line at a constant speed without external force.
(2) use induction to guess.
Students can sum up some experiences and facts, draw conclusions about physical phenomena and processes, and then guess. For example, when people speak, the vocal cords will vibrate; When bees buzz, their wings will vibrate; When you knock on the table, the table vibrates and makes a sound; When plucking the piano, the strings vibrate to make a sound ... Students put forward a conjecture by summarizing a large number of examples: sound is produced by the vibration of objects.
(3) use analogy association to guess
In scientific research, known phenomena and processes are often compared with unknown phenomena and processes to find out their similarities, similarities or connections, and then some characteristics and laws of unknown phenomena and processes are inferred on this basis. In inquiry teaching, we can use this analogy method to make conjectures through association. For example, when exploring the characteristics of current in series circuit, current can be linked with water flow, which leads students to compare circuit to pipeline and current to water flow. Students are familiar with the situation of water flowing in pipes, and thus put forward the conjecture that all currents in series circuits are equal.
There are many ways to put forward conjecture in scientific inquiry, such as observation and analysis, reverse thinking, retrospective judgment, causal judgment, generalization and extrapolation, etc. This requires teachers to guide students appropriately according to the content of inquiry, students' existing knowledge, experience and objective facts in teaching.
3.3 create a harmonious classroom atmosphere, so that students dare to guess.
Rogers believes that "psychological freedom" or "psychological security" is the basic component conducive to creative activities. If a student feels that the psychological atmosphere in the classroom is free and safe, he will feel comfortable, without spending time to protect himself or being afraid of criticism from others. He can always make progress according to his chosen goals, dare to express his views and dare to guess. If our teacher gives students a feeling of "inaccessible and aloof", then even if students have some guesses and assumptions in the learning process, he dare not tell us, and of course, it is impossible to cultivate and train students' guesses and assumptions. Therefore, in inquiry teaching, teachers should treat students' guesses with a developmental perspective, find their bright spots, encourage and praise students more, and treat students' guesses positively, even if they are absurd, rather than cynicism.
3.4 Provide enough time for students to fully imagine.
When guessing, each student has a different view of the problem. When teaching, we should fully spread students' thinking and put forward different guesses. Each student's thinking ability, level and agility are different, and the time required to make a guess is also different; Students should constantly communicate, discuss and even debate when guessing, which also needs to be based on a certain period of time. Therefore, in inquiry teaching, students should be given enough time to give full play to their imagination and put forward various possible conjectures. If there is no certain time guarantee, guessing can only be carried out in a hurry, which can neither let all students guess, nor let the guessing reach its due depth.
3.5 Use brainstorming method to fully demonstrate students' guesses.
Brainstorming is a guessing method, which allows students to guess as much as possible according to their own views on the problem. Before all the possibilities are put forward, teachers and other students should not interrupt and evaluate them. Brainstorming can give full play to students' intelligence, arouse students' initiative and enthusiasm, and let students speak freely and put forward all conjectures. For example, when guessing the factors that affect buoyancy, students suggest that the factors that affect buoyancy may include: the density of the object, the volume of the object, the density of the liquid, the volume of the object immersed in the liquid, and the depth of the object.
4. The strategy of transforming conjecture into hypothesis
In inquiry teaching, due to the limitation of time and conditions, it is impossible to design all conjectures for experimental verification, which requires further processing and transformation of conjectures into scientific hypotheses.
4. 1 Through the experimental verification, the hypothesis is extracted from the conjecture.
After the conjecture is put forward, when it is uncertain whether the conjecture is reasonable, some simple experiments can be used to verify it, and some unreasonable conjectures can be eliminated to form a scientific hypothesis. For example, in the conjecture of exploring Archimedes' principle, whether buoyancy is related to the density, volume and immersion depth of an object in liquid can be judged by the experiment of "weighing method".
(l) The buoyancy of copper and aluminum blocks with the same volume is equal when immersed in water by "weighing method", which shows that the buoyancy has nothing to do with the density of objects.
(2) The buoyancy of the same stone when it is completely submerged in water is different from that when it is partially submerged by "weighing method", which shows that buoyancy is related to the volume of the object immersed in liquid.
(3) The buoyancy of stones immersed in water at different depths measured by "weighing method" is the same, which shows that buoyancy has nothing to do with their immersion depth.
It is concluded that buoyancy may be related to the density of the liquid and the volume of the object immersed in the liquid. To test this hypothesis, let the students do the following experiments:
L) Press the empty mineral water bottle slowly into human water and feel the buoyancy. The buoyancy is related to the volume of the discharged liquid.
2) saltwater floating eggs. Buoyancy is related to the density of the displaced liquid, thus deepening students' perceptual knowledge.
Reuse the following relationships:
G=mg m=ρV G=ρVg
Then row g = row m g = row ρ v row g.
It is further speculated that if buoyancy is related to the volume and density of the displaced liquid, buoyancy may be related to the gravity of the displaced liquid.
4.2 After the rationality analysis, put forward the hypothesis.
Some conjectures, after some analysis and reasoning, can be denied. For example, what is the reason for the formation of circular spots in the shade? Put forward two conjectures. First, the leaf is round and its shadow is round. Because the leaves overlap in the air, so do the circular spots on the ground. Secondly, because the sun is round, the spot on the ground is round, and the round spot is formed by the round sun. Which of these two conjectures is more reasonable? According to the existing knowledge, the shadow is where the light can't shine, and the spot is where the light shines. The circle under the shadow is a light spot, not a shadow. All circular spots on the ground are a standard circle, and only the shape of the sun matches it. Facts support the second guess, so it is unreasonable to draw the first guess through reasoning and analysis. Guess and hypothesis are the key links and important factors in scientific inquiry, and guess and hypothesis ability is an important ability in scientific inquiry, but students' guess and hypothesis ability will not be automatically enhanced, and they need to be guided and trained constantly in combination with their existing education and teaching knowledge and experience.