Xueba's Sea of ​​Stars

Chapter 33 The Preliminaries Begin

At this moment, Wu Bin is already sitting on the bus bound for Xianghai Middle School, which is the venue for this physics competition.

There were ten participating students on the bus, including Wu Bin, and at the moment, Wu Bin, who was sitting alone with a book, seemed a little isolated.

But this is also normal. Among the other nine people, Zhang Qiu is the one who knows Wu Bin the most, but the best relationship between the two is that they are classmates in the same class, oh, they even worked on duty together .

But Zhang Qiu and the other four senior high school students are old friends. They have participated in the competition together since the first year of high school, and they have persisted all the way through gritted teeth. It is an absolute comrade-in-arms friendship.

There are four left, two of them are in the first year of high school, and they are probably going to train for military training, so naturally they sat together. exam questions.

"nervous?"

Seeing Wu Bin sitting alone, Bi Wanqing sat next to him and asked.

"Fortunately, both teachers have such confidence in me, why should I be nervous." Wu Bin closed the book and replied.

Bi Wanqing smiled after hearing this: "Your logic makes sense, so have you completed your plan after passing the preliminaries?"

"Well, I'll try to make up all the other subjects within a week. Anyway, it's no problem to be able to explain to the teachers of each subject."

Wu Bin's study plan has basically been arranged. According to his current progress, it is not difficult to complete, and after passing the physics competition this time, his experience is almost enough to upgrade. By then, the new talent point will allow him to The learning speed is further improved.

Of course, it would be even more perfect if there was another lucky draw.

"Your self-confidence is really... very similar to when I was young."

After speaking, both the teacher and the student laughed.

While the teacher and the student were chatting, the bus arrived at its destination quickly and stopped at the entrance of Xianghai Middle School. After everyone got off the bus one after another, Bi Wanqing finally emphasized the competition and led everyone into the school.

According to the instructions of the staff, Wu Bin quickly found his own classroom, and coincidentally, Zhang Qiu was sitting not far from him.

"come on."

After a word of encouragement to each other, the two sat down.

The next step is to listen to the invigilator read out various disciplines in the examination room and distribute the papers.

As soon as he got the paper, Wu Bin scanned the questions on the paper and finally came to the conclusion that

'funny'

After scanning the questions, Wu Bin can see that the questions on this paper are very polarized. Among the five multiple-choice questions, four of them are "can be answered by people with a little foundation in high school physics."

The first question is a common knowledge question in physics, the second question is a simple heat exchange in thermology, and the third question...is this a Chinese question?
The fourth question is about light intensity, which is beyond the scope of high school. If you haven't studied it, it may be a bit difficult, but if you have studied it, it's trivial.

The only difficult thing is the fifth question. Because the elliptical orbit brings some calculation difficulties and difficulty in thinking, it is a bit tortured, but it is okay for Wu Bin. He is quite good at brain-engaging questions after a lot of hard training.

Then there are fill-in-the-blank questions. One is about simple harmonic motion, using a classical model, which can be easily answered using simple vibration dynamics equations.

The other two fill-in-the-blank tests are optics, a sub-optical axis and a Young's double slit, both of which are basic knowledge in optics.

So far, there are really no particularly exciting questions, and the last one is a big one. The first few questions are not too difficult.

Among them, there are two courses in electricity, one is to turn the coil, the foundation of the basics, and the other is the calculation of the Hall coefficient. Wu Bin has never seen the type of topic, but it is not too difficult. A few troublesome calculations can be done with calculations. Knock it out and get it done quickly.

But the finale mechanics question was really a sudden change in painting style, and it was the most polarized question in Wu Bin's opinion.

The topic is this.

An artificial satellite with a mass of m=3000kg moves in a circle around the earth at a height of H=180km from the ground, where the gravitational acceleration g=9.3m·s minus 2 square is affected by air resistance, and in one year , the altitude of the artificial satellite will drop by ΔH=0.50km.It is known that the resistance F experienced by an object when it moves at a velocity v in a fluid with a density of ρ can be expressed as F=2/1 pACv, where A is the maximum cross-sectional area of ​​the object, and C is the drag coefficient, which is related to the shape of the object ．When the satellite is running at high altitude, it can be considered that the drag coefficient of the satellite is C=1, and the maximum cross-sectional area of ​​the satellite is A=6.0m. The known radius of the earth is R0=6400km.Try to estimate the atmospheric density where the satellite is located from the above data.

This is a final question about universal gravitation, and the requirement is naturally to use the data given in the question to estimate the density of the atmosphere around the satellite.

But this question, in fact, can be regarded as hitting Wu Bin's gun. The difficulty of this question Wu Bin can see is that candidates need to find the correct outstanding elements, ignore secondary factors, and then make a reasonable estimate, that is, the ability to read the question , that is, the battlefield where abstract thinking ability can exert its greatest energy!

And what does this outstanding element refer to?

Since the problem setting requires estimating the atmospheric density where the satellite is located, it is known from F=12pACv, and p is required.

That is to find the relationship between F and v, so obviously, this is a problem of function transformation.

After you have the idea of ​​​​solving the problem, it will be easy, the formula!That's it!
Solution: Assuming that the speeds of satellites before and after 1 year are v2 and v[-] respectively, according to the law of universal gravitation and Newton's law,

GＲ1分之Mm=R1分之v1，（R1＝R0＋H）（1）

ＧR2分之Mm=mR2分之v2（R2＝R0+H-ΔＨ）（2）

With the idea, the formula is listed, and the rest can be pushed, but there is a lot of mechanical knowledge involved, and the process is not easy.

After solving it, Wu Bin let out a long breath.

'Your sister, there are so many pitfalls, it is still hard to learn calculus, otherwise the force F in the question is a variable force, and it can be solved by calculus. '

'However, it may be a pit, anyway, I can solve it. '

After relaxing for a while, Wu Bin looked around and found that many people had already left the classroom that was already full.

'Should I wipe it? ? ?Are the contestants so scary?This question is quite difficult. Can you do it all so quickly?Does this mean that the preliminaries are not difficult for them? '

'It seems that there are a lot of masters in the rematch. '

After thinking about it, Wu Bin looked in Zhang Qiu's direction, and found that he was still there, and he kept scratching his head with his pen, looking uncomfortable.

'It is estimated that I have solved the last question, Amitabha, come on. '

At first, Wu Bin thought about handing in the papers, but when he thought about the pressure he had given Zhang Qiu by handing in the papers this morning, it was okay after thinking about it. Anyway, it’s not too late, so let’s try to use other methods to solve the two electrical problems. up.

(End of this chapter)