I'm a genius star

Chapter 555 Shocked the audience
The female teacher smiled awkwardly, and said to the student: "Student Li, first of all, your hypothesis of piercing the earth is not true. If the earth is pierced, once the center of the earth is pierced, the entire earth will collapse instantly. It's not far from human extinction, so let's not discuss this issue, shall we?"

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

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

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

Su Yun smiled slightly and walked towards the podium. Some students immediately turned on their ipads, ready to take notes, and several students excitedly turned on their phones and started recording videos.

Su Yun stood on the podium, facing everyone, and began to speak unhurriedly: "If we want to discuss this rather 'interesting' question raised by this student, we first need to start from a very simple motion—the theory of simple harmonic motion. rise."

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

"If we fix one end of the spring and tie a small ball to the other end of the spring, when we stretch the spring from the ball end and then release it, the ball will retract to a balance point of the spring, That’s where the original length of the spring is.”

"But at this moment, the ball has been accelerating, so its speed will not be zero. It will continue to rush forward, compressing the spring. When it is compressed to a certain point, it will stop again, and then move in the opposite direction, and so on."

The classmate Li who stopped earlier frowned, and suddenly said: "We all know about simple harmonic motion, but does it have anything to do with the question I raised earlier?"

Others also looked at Su Yun.

Su Yun smiled, picked up the chalk, and began to explain: "Since you know simple harmonic motion, classmate, it's easy to talk about. First, let's take the spring as an example. We have a formula called Hooke's law, F=kx , k is the stiffness coefficient, x is the stretching or compression distance of the spring, and F is the tension or elastic force of the spring."

"At the same time, we can draw another circle, and a small ball performs uniform circular motion in this circle. Then we can actually decompose this uniform circular motion into two simple harmonic motions."

"Through this method, we can calculate the period of simple harmonic motion, which is the time it takes to go back and forth. The time here is limited, so I won't push it to everyone. The conclusion is that the period of simple harmonic motion is the same as circular motion."

"Then this cycle T=2π√(m/k), I believe that you can calculate it easily as long as you do the calculations, so I won't say much here."

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

"Dr. Su, we are not three-year-olds anymore. May I ask what you want to say? We can understand simple harmonic motion. The key is what is the relationship between this simple harmonic motion and the question I raised?"

Li's speech suddenly made several teachers present unhappy.

Obviously you are a student, have you ever talked to the teacher like this?And the person standing on the stage is Dr. Su!As for the question you raised, Dr. Su kindly answered it for you, but you are still so picky!

Several teachers stood up and wanted to warn the student, but they stopped them with their hands.

"Then, as this student said, after we all understand this simple harmonic motion, we can study the issue of the hole in the earth." 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, the revolution of the earth, or the change of the density of the earth. And it doesn’t take into account that there is magma in the earth, nothing.”

"Let's just dig a hole from one end of the earth to the other. The hole is very, very small. Well, what would happen if a person jumped into the hole?"

Tu 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 subjected to the gravitational force of the earth, and how to calculate the magnitude of this gravitational force, we can know it according to the physical formula."

"At this time, the gravitational force of the earth is divided into two parts. One part is the gravitational force on the human being under the person, that is, the part that removes the center of the earth's crust, and the other part is the gravitational force on the human being from the shell above the human .”

"But the resultant force of the outer shell is actually zero, and the gravitational force on people is actually given by the small ball inside the earth."

Tu Su was slightly taken aback, apparently this was the first time he had heard of this thing.

Isn’t the gravitational force that the earth exerts on humans?How come that little ball in the earth turned against people again?

However, a professor from the physics department behind nodded and said: "Dr. Su is right. The gravitational force of the earth is mainly caused by the small ball inside the earth. The gravitational force on 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!He's not talking nonsense!

The classmate Li in the front row listened very seriously and didn't make any more noise.

Su Yun began to write on the blackboard: "Then let's assume that the radius of this small ball is r, and now I want to calculate how much gravity a person is subjected to. According to the formula of universal gravitation, F=G·Mm/r^2, where M is the mass of the small ball in the earth, and r is the distance from a person to the center of the earth."

“那么我们还知道地球里面的这个小球的质量M就等于地球的密度，乘以里面这个小球的体积V，小球的体积V=4/3πr^3，所以小球的质量为M=ρ·4/3r^3。”

"We substitute this formula into the above formula of universal gravitation, and we get: F=4/3·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, "You should have discovered something, right?"

Li, who was in the front row, looked at the draft in his hand in surprise, and murmured: "Hooke's Law, F=kx!!"

Some people still didn't respond.

"What's the situation? Why is Hooke's Law involved?"

"Oh, didn't you find out? F=4/3·Gmr, where 4/3·Gm is a constant, G is the gravitational constant, m is the mass of a person, they are all constant, so it is equivalent to return At F=kr, and r happens to be the distance!"

"Fuck!"

"Is this all right?"

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

"It's too awesome."

"Shh, don't talk, and then, and then, Dr. Su, please talk!"

The old professors from the physics department in the back couldn't help but shook their heads with a wry smile.

Usually they are in class, and the students are almost doing it by themselves. Few students will follow their rhythm. After all, most of the people who come here are geniuses. If you study by yourself, you can completely solve all the problems in the books. Just gone.

Like this, it is almost impossible for everyone to follow the teacher's thinking and eagerly urge the lectures to happen in their classrooms.

Su Yun smiled lightly, and continued to explain: "Here we can see that the gravitational force F on a person is only related to the distance between the person and the center of the earth, and they are directly proportional."

"So, this is the spring problem we just talked about, which is the simple harmonic motion we are familiar with!"

"So this person will accelerate to the center of the earth, then decelerate to reach the other end, just when the speed becomes zero, then accelerate in the opposite direction to the center of the earth, and then decelerate in the opposite direction to the starting point. This back and forth movement is a cycle."

explained.
This girl really explained it!

Several students in the audience were speechless.

"Is this all right?"

"I knelt down to Dr. Su."

"Earth can be solved even if it is broken down, Dr. Su is going to heaven!"

"I won't obey anyone in my life, but I will obey Dr. Su!"

"Awesome..."

(End of this chapter)