1. What is a physical model?
The physical model is based on grasping the main factors and ignoring the secondary factors. It can reflect the essence and theme of things concretely, vividly and profoundly. There are two situations: one is the motion model. When studying the mechanical motion of an object, the motion is often very complicated, and it is impossible to have simple linear motion, uniform motion and circular motion. In order to make the research possible and simplify, some secondary factors are often ignored and the problem is idealized, such as introducing idealized motion models such as uniform linear motion, uniform variable linear motion, uniform circular motion and simple harmonic motion. The other is the solid model representing the research object, such as particle, simple pendulum, ideal gas, point charge, point light source, nuclear model and so on. Take the particle as an example, it is an idealized physical model, which highlights the quality of the object and abandons the shape, volume, temperature, luminescence, electrification and other factors of the object. ?
Second, the role of the physical model?
1. simplify. Any practical problem is concrete, vivid, complex and changing. Turning practical problems into idealized physical models will simplify complex problems and clarify vague ones. ?
2. similarity. Classicism is also called representativeness. The process of establishing an idealized physical model is the process of classifying things. An idealized physical model is a representation of things with the same properties. Using the similarity of idealized models and transplanting analogy and thinking methods, some difficult problems can be skillfully solved. ?
3. variability. When dealing with practical problems, the purpose of transforming practical problems into physical models is to apply physical laws. Different physical laws need different physical models, so things with inherent multiplicity should be transformed into different physical models. ?
4. Be unique. When solving problems, if we can migrate the best problem-solving model, we can skillfully use the conclusions of relevant models and greatly shorten the problem-solving time. ?
For example, as shown in figure 1, the metal bar ab slides down the smooth arc parallel track from the height h, enters the horizontal part of the smooth track, and moves from top to bottom in a uniform magnetic field. In the horizontal part of the track, there is another metal strip cd, called mcd? = 12mab？ =m, if the rod ab never collides with the rod cd, find the final speed of the two rods and the electric energy consumed in the whole process. ?
Analysis: the physical model of this question is: in the process of ab sliding from high H to horizontal orbit, only gravity does work and mechanical energy is conserved. Ab slides on the horizontal track to cut the magnetic induction line, and the current is induced in the loop. Ab is decelerated by Ampere force to the left, and cd is accelerated by Ampere force to the right. Finally, when the speed of the two rods is the same, the induced current disappears and the ampere force disappears. In the process of moving on the horizontal track, the ampere force on the two bars is equal and opposite. Although there is no collision between the two bars, it is a physical model of completely inelastic collision. According to mab? gh= 12mab？ v？ 2、mab？ v=(mab？ +mcd？ ) v' obtains the same speed, that is, the final speed of the two bars v'=232gh. The whole process is a physical model of energy conservation, that is, the loss of mechanical energy is equal to the work done by induced current and also equal to the consumed electric energy, so there is W=? Δ? E=mab？ gh-？ 12(mab？ +mcd？ )v′2 = 23 mgh .？
Enlightenment: The establishment of physical model must be based on the correct analysis of specific physical processes. If the process analysis is unclear and conclusions are drawn blindly, the wrong physical model will be established, which will lead to the wrong problem solving. ?
Change: If the horizontal trajectory in the question is changed into two parts long enough, the width ratio is 2∶ 1, ab always moves in a wider trajectory, and cd always moves in a narrower trajectory, the final speed of the two sticks can be obtained. ?
3. What are the common important models in middle school physics?
There are many pairs of physical models in middle school physics: planetary model and binary star model, traveling wave model and standing wave model, sphere model and cube model, constant power start model and constant acceleration start model. Take the planetary model and binary star model as examples to illustrate. ?
1. Build the model. As shown in the figure, the planetary model is that the mass m of the central celestial body is much greater than that of the planets orbiting it. For example, if the earth moves around the sun, it is necessary to ignore the rotation of the earth, treat the elliptical orbit of the earth as a uniform circular motion, and establish a planetary model. The binary star model consists of two planets that are close to each other. The linearity of each planet is much less than the distance between the two stars, and the binary star system is generally far away from other celestial bodies. Under the action of mutual attraction, they make uniform circular motion with the same period around a point on the connecting line, as if they were connected by an invisible rod. This is a binary star model. ?
2. Important relationships. The basic relationship of the planetary model is:?
GMmr？ 2=mv？ 2r=m？ 2? π? T2r？
What is the basic relationship of binary star model?
GM 1？ m？ 2r？ 2=M？ 1? 2? π? T)2r？ 1=M？ 2? 2? π? T2r？ 2。 ?
3. Unification of models. Double star model r? 1r？ 2=M？ 2M？ 1，r？ 1+r？ 2=r, so there is R? 1=M？ 2M？ 1+M？ 2r，r？ 2=M？ 1M？ 1+M？ 2r, when m? 1? m？ 2, namely M 1? When it is a central celestial body, r? 1→0，r？ 2≈r, this is a planetary model. ?
(Author: Guangxi Fangchenggang High School)