The female teacher smiled awkwardly and said to the student: "Student Li, first of all, your assumption that the earth will be penetrated is indeed not valid, if the earth is penetrated, once the center of the earth is penetrated, the entire earth will collapse instantly, and it is not far from the extinction of human beings, let's not discuss this issue for now, okay?"

That classmate Li looked at Su Yun, and although his face was very reluctant, he still said, "Okay." Then he picked up the book in his seat and turned to leave.

However, Su Yun suddenly said at this time: "Actually, we can still discuss this issue roughly. ”

Su Yun's words immediately attracted everyone's attention, and the classmate Li who was about to leave before also stopped his tracks.

Su Yun smiled slightly and walked towards the podium, some students immediately turned on their iPads and prepared to take notes, and several students turned on their mobile phones excitedly and began to record videos.

Su Yun stood on the podium, faced everyone, and began to say unhurriedly: "If we want to discuss this more 'interesting' question raised by this classmate, we first need to start with a very simple movement - simple harmonic movement. ”

"I believe everyone should be familiar with simple harmonic motion, let's first talk about the simplest simple harmonic motion, that is, the spring we often discuss. ”

"If we hold a section of the spring in place and tie a ball to the other end of the spring, when we stretch the spring from the end of the ball and then release it, the ball will retract to a point of equilibrium in the spring, which is where the spring was originally long. ”

"But at this point the ball is accelerating all the time, so it's not going to have zero velocity, it's going to keep rushing forward, compressing the spring. After a certain point of compression, it will stop again, and then move in the opposite direction, and so on. ”

The student Li, who had stopped before, frowned, and suddenly said, "We all know about the Jian Harmonic Movement, but does this have anything to do with the question I raised earlier?"

The others also looked at Su Yun.

Su Yun smiled, picked up the chalk, and began to explain: "Since you know the Jian Harmonic Movement, it's easy to say. First of all, take a spring as an example, we have a formula, called Hooke's law, F = kx, k is the stiffness coefficient, x is the distance of the spring elongation or compression, and F is the tension or elasticity of the spring. ”

"At the same time, we can draw another circle, in which a small ball moves in a uniform circular motion. In fact, we can decompose this uniform circular motion into two simple harmonic motions. ”

In this way, we can calculate the period of simple harmonic motion, which is the time it takes to go back and forth. Time is limited here, so I won't push it to you, and the conclusion is that the period of simple harmonic motion is the same as that of circular motion. ”

"Then this period T = 2π√(mk), I believe that as long as you calculate, it should be easy to deduce this, so I won't say much here. ”

The classmate Li had already sat back down, but his face was obviously impatient all the time.

"Dr. Su, we are not three-year-olds anymore, what do you want to say? We can all understand the simple harmonic movement, the key is what does this simple harmonic movement have to do with the question I raised?"

Li's speech immediately made several teachers present unhappy.

Obviously, you are just a student, do you talk to the teacher like this? And it is Dr. Su who is standing on the stage! As for the question you raised, Dr. Su kindly gave you an answer, and you are still eating inside and out!

Several teachers stood up to warn the student, but reached out to stop them.

Then, as this student said, after we all understand this simple harmonic motion, we can study the problem of this earth burrowing. Su Yun smiled and didn't care about Li's speech.

Su Yun drew a circle on the blackboard with chalk and said, "First of all, let's take a look, if this is an earth, we don't consider the rotation of the earth for the time being, nor the revolution of the earth, nor the change in the density of the earth, and we don't consider the magma in the earth, we don't think about anything." ”

"We're going to make a hole from one end of the earth to the other, and it's very, very small. Okay, so what happens if a guy jumps into the hole?"

Yan Su looked at the blackboard thoughtfully, and many students couldn't help but start thinking.

Su Yun continued: "Let's see, after this person jumps into the hole, he will be affected by the gravitational pull of the earth, and how to calculate the magnitude of this gravitational force, we can know according to the physical formula. ”

At this time, the gravitational pull of the earth should be divided into two parts, one part is below the person, that is, the gravitational attraction of the part of the earth's crust on the person, that is, the part of the crust above the person, and the other part is the gravitational pull of the shell above the person on the person. ”

"But the net force of the outer shell is actually zero, and the gravitational pull on people is actually given by this small ball on the earth. ”

Yan Su was slightly stunned, obviously the first time he had heard of this thing.

Isn't the gravitational pull of man the same as the earth to man? How did it become the ball in the earth against man?

However, a professor from the physics department in the back nodded and said, "Dr. Su is right, people are affected by the gravitational pull of the earth, mainly the small ball inside the earth, and the gravitational force of the outer shell of the earth is actually very small, almost zero. ”

After an old professor spoke, everyone couldn't help but feel ashamed, what Su Yun said was true!

The student Li in the front row listened very seriously and did not make any more sound.

Su Yun began to write on the blackboard: "Then let's assume that the radius of this ball is r, and I now want to calculate how much gravitational force people are subjected to." According to the formula of gravitational force, F=G· Mmr^2, where M is the mass of the ball in the earth and r is the distance from man to the center of the earth. ”

"Then we also know that the mass M of this ball in the earth is equal to the density of the earth, multiplied by the volume V of the ball inside, the volume of the ball V = 43πr^3, so the mass of the ball is M = ρ·43r^3. ”

"If we substitute this formula into the gravitational formula above, we get: F = 43·πρGmr. ”

Su Yun's words came to an abrupt end here, and everyone looked at the formula on the blackboard in surprise.

Su Yun smiled slightly, "Everyone should have discovered something, right?"

Student Li in the front row looked at the sketchbook in his hand in surprise and muttered, "Hooke's law, F=kx!!"

Some people still haven't reacted.

"What's going on? Why is it going to Hooke's Law again?"

"Well, didn't you notice?F=43· GMR, of which 43· Gm is a constant, G is the gravitational constant, m is the mass of a person, all of which are constant, so it is equivalent to returning to F=kr, and r is just the distance!"

"it!"

"Is this all right?"

"Khan, my mom asked me why I was kneeling in class. ”

"That's awesome. ”

"Shhhh

The old professors of the physics department in the back couldn't help but smile wryly and shook their heads.

Usually they go to class, and the students almost do it themselves, and there are very few students who can follow their rhythm, after all, most of the people who come here are geniuses, and if you study by yourself, you can completely solve all the problems in the books.

In this way, everyone follows the teacher's thoughts, and they are also urging the lecture with great interest, which is almost impossible in their class.

Su Yun smiled faintly and continued to explain: "Here we can see that the gravitational force F experienced by people is only related to the distance between man and the center of the earth, and they are directly proportional to each other. ”

"So, this is the spring problem we just talked about, which is what we know as simple harmonic motion!"

"So this person, will accelerate to the center of the earth, then slow down to the other end, just when the speed becomes zero, and then reverse the acceleration of the center of the earth, and then reverse the deceleration to the starting point, this back and forth movement, is a cycle. ”

Explained out...

This girl really explained it!

Several students in the audience were speechless.

"Is this all right?"

"I knelt for Dr. Sue. ”

"The earth can be solved if it is broken, Dr. Su is going to heaven!"

"I don't accept anyone in my life, so I will serve Dr. Su!"

"Awesome. "I am a genius star, the latest chapter of "I am a genius star, claw bookhouse" is free to read for the first time.