About the Author
Zhang Zeji, a 2020 graduate of the High School Attached to the National People's University, a 2020 undergraduate of Yuanpei College, Peking University.
Write in front
In a sense, physics, chemistry, and biology are three disciplines that progress from the virtual to the real, layer by layer: Physics is the most abstract and general, and it often uses a few simple axioms to derive the evolution of everything in the universe; chemical comparison Specifically, it is necessary to explore the commonalities and characteristics of atoms and molecules one by one. Taxonomy is also one of the commonly used methods in chemistry; biology is the most specific and trivial, involving the function of each smallest structure and the complexity between them. When learning biology, it is inevitable to memorize a lot of knowledge points. Each of these three subjects has its own uniqueness, and therefore each has its own learning method. If you expect to learn biology like physics, abandon textbooks and use pure logic to deduct, you are often confused and have no gain; and if you learn all the formulas and models of physics, you often pay a lot of money. Efforts, but the gains are very limited. This time I will mainly share some understanding of high school physics learning.
Pay attention to qualitative analysis
Qualitative description is an important part of solving physical problems, that is, to make a rough judgment on the changes of the physical system before specific calculations. For example, a free-falling weight, two small balls colliding elastically, and an electron moving in a circular motion in a magnetic field, we often have a clear physical picture of these simple physical situations, and we have an intuitive understanding of the effects of related forces. Therefore, once the calculation of the relevant situation is involved, we can often get ideas quickly. Corresponding to a complex system, we are required to put aside specific calculations for the time being, and by analyzing the causal relationship between physical quantities, we can obtain the general situation of the changes in the entire system step by step, thereby forming a more intuitive image in our mind. Qualitative descriptions often play the role of outlines. Once we have a clear physical picture, we will more easily grasp the key to the problem, understand what equations should be listed in order from the beginning to the end, and what unknown quantities can be found to get the final answer.
Example: An unpowered aircraft with a mass of M0 and a muzzle velocity of v0 is moving in space dust. During the movement, the aircraft will absorb the dust. The adsorption mass is proportional to the distance, and the proportional coefficient is a constant α.
(1) Determine the total distance traveled by the aircraft before stopping;
Let's think about the whole process: the spacecraft moves in a straight line in the space dust, and every time it moves forward, it will adsorb a little space dust-the adsorption mass is proportional to the distance, which means that the space dust along the road is evenly distributed-adsorbed The space dust will get the same speed as the spacecraft, and the spacecraft will be dragged, and the speed will inevitably decrease. The further the distance travels, the lower the speed will be, but it will never decrease to zero. This is because the effect of the dust and the spacecraft is equivalent. As a result of a completely inelastic collision, the overall momentum is conserved, so the spacecraft always has a positive speed.
At this point, we have got the picture of the problem: the spacecraft absorbs sand and dust to slow down, and the speed eventually approaches zero infinitely. List the momentum conservation equation:
This question itself is not difficult, but it can be seen from it that qualitative analysis is one of the important ways for us to understand physical processes, and it can often intuitively prompt us which conserved quantities to use and write those equations. In more complex situations, the importance of qualitative analysis will continue to increase.
Pay attention to the "initial" volume
For more complex physical systems, the most fascinating, and sometimes also the most annoying, is that various physical quantities are often entangled. You have me in you, and you in me, which move the whole body together. The overall look is often entangled. Confused, don't know where to start. At this time, an effective way is to look back and find the physical quantity that changed in the first place, and then proceed from this to derive the causal relationship between the various physical quantities.
Take the 16th question of the 2020 high school midterm exam as an example:
First of all, we can judge that what changes first is the force on the motor rope. The effect of force is to bring about acceleration. Therefore, a small object will be accelerated only by this pulling force. So, the next question is, will the wood accelerate? It is easy to know that this depends on the maximum static friction provided by the ground. To figure this out, the whole system will be solved.
It can be said that finding the physical quantity that changes first is equivalent to finding a thread, along with it, you can peel off the cocoon little by little to show the change of the entire system. (In fact, many physical systems are similar to a "function". Given the initial value conditions, it is equivalent to a given input. The output of the "function" is the value of each physical quantity at each moment.)
Hold on to Conserved Quantity
Theoretically, we only need Newton's three laws, plus the properties of gravitation, Coulomb's force and other forces, and we can quantitatively find the state of any system at any time, but in fact it is difficult for us to achieve this. One of the reasons is that some physical processes are too complicated, involve too many changes and details, making accurate description almost impossible, and often involve mathematical tools such as calculus.
A classic example is the gravitational slingshot:
If the conservation of momentum is not applied, it will inevitably involve forces whose calculation direction and magnitude are constantly changing. It is conceivable that the solution of this process will become extremely difficult. The emergence of conserved quantities allows us to find invariants in the complex changes of the system, so we can directly correlate the initial and final states of the system, thus avoiding complicated calculations for intermediate processes. At the same time, conservation of momentum and conservation of energy are also the key content of high school learning, so when propositions are often focused on investigation, therefore, conservation of quantity has become an important breakthrough in some topics.
However, everything is difficult to be perfect, and the conserved quantity undoubtedly greatly simplifies our calculations, but relatively, its application conditions are limited. Momentum conservation requires the system to be free from external forces, or the combined external force is zero; while energy conservation requires the system to be free from external forces, or the sum of the work done by the external forces is zero-pay special attention to the difference between the combined work of external forces and the sum of work done by external forces. These conditions and details need a little attention when reviewing.
Some tips:
Think about the same physical phenomenon from different angles, try to use different methods to solve the problem, and then experience the inner connection of different methods, and deepen the understanding of physical concepts.
Pay attention to the physical picture and the foundation of mathematics. It is necessary to know how to describe a physical process in mathematics, but also to think about the mathematical formula written down, and what the physical image corresponds to the mathematical result obtained.
Students who have the ability to learn can have a little knowledge of the theorem of centroid motion and Koenig's theorem. It will not be very complicated, but it is very helpful to simplify the calculation for understanding the collision process.
Finally, I wish everyone a smooth physics study!
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