Shulou(Shulou.com)11/24 Report--

Original title: "Why do apples fall, but hot air balloons fly to the sky?"

Everything in the universe is in motion and change. Physics is to study the laws of their motion and change, why and how they move.

We see objects because photons come into our eyes; we hear sounds because sound waves pass through the air into our ears; we answer phones because electromagnetic waves send messages to us; and apples fall when they are ripe, not to mention chairs move when pushed.

If there were no motion, the world would be dead, and there would be no physics.

Motion is so common and obvious, but why do objects move? At first glance, this question may seem funny, but on second thought, you will find that it is far less simple and natural than you think.

Why do apples fall and hot air balloons fly? I push the chair, and the chair moves forward. As soon as I let go, the chair stops. Is there an external force to move the object? An iron ball falls faster than a feather because it is heavier? These questions are so common, but they are extremely difficult to answer. Many natural philosophers in ancient Greece pondered these questions, but the answers were unsatisfactory.

For example, you see, I push the chair and the chair moves. This is easy to understand--transmitting force through contact is also easy to accept. But when the apple falls, nothing touches it. Why does it move? When the balloon rises, nothing touches it. Why does it move? And why does the apple go down while the balloon goes up? Could it be that heavy things fall and light things fly into the sky?

See here, some students must want to say: apples fall because of the downward gravity, hot air balloons rise because of the upward buoyancy. Many parents also like to throw their answers directly when answering their children's questions.

The answer was correct, but it fell from the sky. Children can only get a fragmentary knowledge point through this answer, and they cannot understand the knowledge system behind it, nor can they understand how science is established. The ancient Greeks made a careful analysis and deep philosophical thinking of nature, and finally formed a set of self-consistent natural philosophy system.

Aristotle is at the heart of this process, and we shall call this whole set of views on the world the Aristotelian worldview.

This view holds that the earth is the center of the universe and that the sun, moon and stars revolve around the earth. Matter on earth consists of four basic elements: water, fire, earth and air. Earth naturally moves toward the center of the universe (so stones fall); water naturally moves toward the center of the universe, but less so than earth (so water also moves downward but above earth); gas naturally moves toward water and above earth (so bubbles in water rise upward); and fire has a natural tendency to move away from the center of the universe (so fire burns upward in air).

An object tends to come to rest either because its constituent elements have reached their natural positions in the universe (e.g., water and earth at the center of the earth) or because something else (e.g., the surface of the earth) is blocking it. A stationary object will remain stationary unless it has some other source of motion (either its own motion toward its natural position in the universe, or an external force, such as pushing a table).

I won't list other views one by one. What do you think after reading them? Do you get the feeling that although these ideas may seem "childish" today, they are a self-consistent system? It can round out its own words without contradicting itself; it can also explain why objects move, and it can better explain various phenomena seen by the ancients. Even for children, this set of theories is more in line with "common sense" and easier to understand and accept. But this was not science, but natural philosophy, and true science had not yet been born. The Aristotelian worldview would dominate Europe for nearly 2000 years, until Galileo appeared.

Galileo's discovery Galileo believed that not only qualitative analysis of motion, but also quantitative calculation. Instead of discussing metaphysical, unquantifiable things such as the purpose and nature of objects, the motion of objects should be described quantitatively mathematically and verified experimentally.

This meant that Galileo abandoned the tradition of natural philosophy since ancient Greece and formally founded modern science based on mathematics and experiment, with the task of "describing natural phenomena" rather than trying to "explain the essence of natural phenomena." The weight will fall, then see how it falls, how much it falls in the first second, how much it falls in the second, and find the pattern.

It is said that the heavier an object, the faster it falls. Let's experiment and see if this is true of the ratio of a heavy iron ball to a light iron ball. It is said that all objects tend to come to rest when they reach their natural position. Let's experiment to see if this is true.

After a series of experiments, Galileo was shocked to find that things were not what he had thought. It feels unreliable, and you have to experiment.

First of all, Galileo discovered from a series of slope experiments that whether an object moves or not is not directly related to whether it has a force or not. The motion does not need external forces to maintain it. He designed a smooth slope and found that no matter how high the ball was dropped from the left, it would reach almost the same height on the right.

Next, reduce the slope to the right so that the right becomes flatter and flatter. Then, to reach the same height, the ball has to travel a greater distance.

Finally, the slope on the right side is completely flattened, and the right side becomes a plane, and the height is always the same. Thus, no matter how long or how far the ball travels, it is impossible to reach the same height as the left side. Never reaching the same height on the left means that the ball will continue to move in a straight line at a constant speed (assuming the ground is perfectly smooth). It's like skating at an ice rink. The smoother the ground, the farther you can skate at once. If the ground were perfectly smooth, one would never be able to stop until he hit another obstacle.

Through this experiment, Galileo discovered that motion itself does not require force to maintain, and that objects can remain in uniform linear motion without any external force. So what is the function of force? Pushing the chair hard did indeed change the state of the chair, and indeed it seemed that the harder one pushed, the faster the chair moved.

Galileo investigated these questions further and found that forces are not the cause of maintaining motion, but of changing the state of motion. In other words, no force is needed to maintain the motion of an object, but force is needed to change the motion of an object, and force is still very useful. Small steel ball on the absolutely smooth ground can always do uniform linear motion, speed and direction are unchanged. But if you push the ball hard, the ball's velocity changes. Galileo's work is very important. He not only pioneered science in the modern sense and pointed out the basic methods of scientific research, but also personally discovered the basic laws of motion of a large number of objects and pointed out the direction for future generations.

Source: "What is high school physics" Author: Long Tail Jun Tsinghua University Press Part of the source network copyright belongs to the original author All editors: Zhang Runxin ★ Author Introduction ★

Long Tail Jun, formerly known as Zhang Wen, began to publish popular science articles on relativity in May 2018 under the name of "Long Tail Science and Technology." Since then, with the Feynman-style learning attitude of "trying to popularize what you want to learn, only when you explain it clearly to others can you understand it," he went further and further on the road of "promoting learning by popularizing science." Long Tail Jun focuses on physics, mathematics, philosophy and science education for primary and secondary school students. He insists on explaining complex scientific problems clearly with extremely popular language and meticulous logic, hoping that more people can feel the beauty of science.★ Introduction ★

It covers almost all the knowledge points of high school physics, tells the ins and outs of these contents smoothly and interestingly, and strings them together in a clear vein to help you construct a physical picture in your mind, so that you can look up and overlook the whole high school physics and have a feeling of condescending. Aerial cognition, physical thinking and overall view allow you to understand the essence of physics, establish the underlying thinking logic, and form a unified thinking path for solving problems. To reach the analogy bypass, do the problem conveniently pick up. For the students who are confused about learning high school physics, they point out the direction, so that learning high school physics will be more handy. I believe you will have a feeling of enlightenment after reading the whole book, not only saying,"Oh, physics is like this, it's not that difficult... "If you are a high-scoring student: After reading and understanding" What is High School Physics ", the benefits lie in the reinforcement and strengthening of the correct subject vision, thinking mode and knowledge system. If you are an intermediate student: master most of these knowledge points. If we can further summarize, compare textbooks, outline, understand the logical main line in the middle, and establish the knowledge system constructed in the book, we can completely rush to high marks. If you still have a lot of room for improvement: read through this book with your parents or teachers, it's best to brush it three times and believe that you will "open up" physics, and steadily improve your grades will be in the future. This article comes from Weixin Official Accounts: Origin Reading (ID: tupyread), Author: Long Tail Jun, Zhang Runxin

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