Is relativity the difference of reference objects?

The relativity principle of mechanics is not included in the middle school physics syllabus, so middle school teachers generally don't need to introduce it to students. The principle of relativity is related to the inertial system. In classical mechanics, the acceleration of the same object is the same for different inertial systems. The simple proof is as follows: Let the velocity of a moving object relative to the S inertial system be U, the velocity of another inertial system S ′ be U ′, and the velocity of S ′ be V, then it is written by the classical velocity synthesis theorem (which can be derived from galilean transformation's formula): U = U ′+V. ① This shows that the position and velocity of the same object relative to the S system and S ′ system are different, but their accelerations are the same. Moreover, in classical mechanics, the mass of the same object is a constant, so the interaction force between objects is the same for different inertial systems. Therefore, as long as Newton's second law f=ma holds for a certain inertial system, it must hold for any other inertial system. In other words, the laws of mechanics are the same in all inertial systems, that is to say, any mechanical experiment done in one inertial system cannot determine whether this inertial system is moving relative to other inertial systems. This principle of mechanical relativity can be deduced from galilean transformation's formula and Newton's second law. It shows that all inertial systems are equivalent from the mechanical point of view, and no inertial system is superior to other inertial systems. The discovery of the principle of relativity in mechanics was first obtained by Galileo through observation and experiment. Galileo wrote in the book "Dialogue between Ptolemy and Copernicus": "Put you and some friends in the main cabin under the deck of a big ship, and then let you take away some flies, butterflies and other small flying insects." Put a big water bowl in the cabin, put some fish in it, and then hang a water bottle to let the water drop into the jar with a wide mouth below. When the ship is at rest, you should pay attention to the insects flying in all directions at the same speed in the cabin, and the fish are swimming in all directions at will and dripping into the jar below. When you throw anything to your friend, as long as the distance is equal, you don't have to use more force in one direction than in the other ... the jumping distance is equal. When you carefully observe these things (although this will undoubtedly happen when the ship stops), and then let the ship move at any speed, as long as the movement is uniform and does not sway from side to side, you will find that the above phenomena have not changed at all, and you can't determine whether the ship is moving or standing still from any of them ... "Paying attention to observation and experiment is one of Galileo's contributions to science. Therefore, people also call this relativity principle of mechanics Galileo's relativity principle. However, people often mistakenly think that this quotation is Galileo's description of the relativity of motion, which leads to the obvious distinction between relativity and relativity. What's more, the reading material Galileo on the relativity of motion and stillness actually moved this quotation out completely. The editor said very seriously, "Galileo of Italy was the first person to discuss the relativity of motion and stillness in depth." It vividly describes that it is impossible to determine whether an object is moving without selecting a reference object. "Then I quote Galileo. In my opinion, no junior two students can understand the meaning of this long text. The editor wants the students to read these reading materials. You can better understand the relativity and reference objects of motion mentioned in the last section "Motion and Static". But the actual effect will only lead students not to read. On the contrary, they are at a loss. In any case, they can't relate Galileo's wonderful description to the content in the textbook, because Galileo's theory here is the relativity principle of mechanics. It is normal for you to see insects flying, fish swimming, people jumping and smoke rising in the cabin, simply because it is assumed that the ship is stationary or "as long as it moves at a constant speed, it will not turn left or right", because such a ship is an inertial system. It is a relatively simple frame of reference problem. The motion of natural objects is absolute, but the description of motion can only be relative. When we study the motion of an object, we must clearly point out which frame of reference the motion is relative to. Otherwise, the motion of the object cannot be determined. This is the relativity of motion. The relativity of motion is obvious. It has been vividly described in China for a long time. Among the Dunhuang lyrics originated in the late Tang Dynasty, there is such a poem "Bodhisattva Man": "Set sail with five poles flat. "This is a boat trip. It's like watching the mountains come and go, but looking at the mountains carefully is a boat trip. " How vividly and interestingly this word describes the relativity of motion. "Meet the Buddha when you see the mountain" is obviously based on the conclusion of the ship. "Looking closely at the mountains, it is a boat tour." This is the reference system of this mountain. After the author gave a wonderful description of the relativity of motion written by the ancients, I went on to say that relativity is the principle of relativity. I said that Dunhuang Quzi Ci originated in the late Tang Dynasty more than 700 years ago, while Galileo was a man more than 200 years ago. It can be said that China people discovered the relativity of sports 500 years before Galileo. It can be seen that many people confuse relativity with relativity principle. To tell the truth, I made such a joke. This is very undesirable. In fact, there are many descriptions of lenses in the theory of motion relativity in ancient works. As early as the Southern and Northern Dynasties more than 1,400 years ago, there was an excellent Zen master, Shan Hui. One of his Zen poems had two lines: "People cross the bridge, but the bridge does not flow." This seemingly absurd statement has split his profound Zen theory. From today's mechanical point of view, it is nothing more than taking the running water under the bridge as the frame of reference. When it comes to the description of the phenomenon of relativity in ancient works, we can't help but mention Chen, a poet in the Song Dynasty. In his poem "On the Road of Fragrance", he said: "Flying flowers shine on both sides of the boat, and the wind blows a hundred miles of embankment for a long time. I don't know if the clouds are moving. " The man standing on the ground thinks that the wind blows the clouds, why is he on the ship? It turns out that he takes the ship as a reference. At this time, the ship is moving from west to east at a speed similar to that of the cloud, so the cloud is still relative to the ship. It is most appropriate to compile some reading materials about the relativity of motion and stillness for students, which can clearly explain the problem and is very interesting. Intentionally or unintentionally, it infiltrated the education of students' love for China culture. In my opinion, middle school physics textbooks should be a way for students to accept relevant knowledge.