Chapter 33 The Preliminaries Begin
At this moment, Wu Bin was already sitting on the bus bound for Xianghai Middle School, which was the venue for this physics competition.
There were a total of ten participating students on the bus, including Wu Bin. At this moment, Wu Bin, who was sitting two seats alone with a book, seemed a little isolated.
But this is normal. Among the other nine people, Zhang Qiu is the one who is most familiar with Wu Bin, but the best relationship between the two is that they are classmates in the same class, oh, and they even worked together on duty. .
But Zhang Qiu and the other four high school seniors are all old friends. They have participated in competitions together since the first year of high school, and they have persisted all the way through gritted teeth. They are absolute comrades.
There were four left, two were freshman in high school, probably going to practice military training, so they naturally sat together. The other two, sophomores in high school, had initially established a comradeship, and were chatting about what would probably happen this time. Exam questions.
"nervous?"
Seeing Wu Bin sitting alone, Bi Wanqing sat next to him and asked.
"Fortunately, both teachers have so much confidence in me, so 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 made plans after passing the preliminaries?"
"Well, I'll try to make up for all the other subjects within a week. Anyway, it won't be a problem if the teachers of each subject can give me instructions."
Wu Bin's study plan has basically been arranged. According to his current progress, it is not difficult to complete. Moreover, after passing this physics competition, his experience will almost be enough to upgrade. By then, the new talent points will allow him to The learning speed is further improved.
Of course, it would be even more perfect if there could be another lottery.
"Your confidence is really... very similar to when I was young."
After saying this, both teachers and students laughed.
While the teachers and students were chatting, the bus quickly arrived at its destination 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.
Following the instructions of the staff, Wu Bin quickly found his own classroom, and coincidentally, Zhang Qiu was sitting not far from him.
"come on."
The two encouraged each other and sat down.
The next step is to listen to the invigilator read out various examination room rules and hand out papers.
As soon as he got the paper, Wu Bin scanned the questions on the paper and finally came to the conclusion.
'funny'
After scanning the questions, Wu Bin could see that the questions on this paper were very polarizing. Four of the five multiple-choice questions were "can be answered by people with a little basic knowledge of high school physics."
The first question is a general knowledge question in physics, the second question is a simple heat exchange in thermal science, 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 before, it might be a bit difficult, but if you have studied it, it’s a piece of cake.
The only difficult question was the fifth question. Due to the elliptical orbit, it brought some calculation difficulties and thinking difficulties, which was a bit torturous, but it was okay for Wu Bin. He was quite good at the mental questions after practicing hard.
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 questions are about optics, a secondary optical axis and a Young's double slit, which are all basic knowledge in optics.
Up to this point, there are really no particularly exciting questions. The final questions are the big ones, and the few questions you can get started with are not too difficult.
Among them, there are two electrical subjects, one is about rotating coils, which is the basis of basics, and the other is about the calculation of Hall coefficient. Wu Bin has never seen the type of question, but it is not too difficult. Several troublesome calculations are done with calculations. Once the tool is knocked out, it will be done quickly.
But the final mechanics question was a sudden change in style, which Wu Bin felt was the most difficult question in the paper.
The title is this.
An artificial satellite with a mass of m=3000kg moves in a circle around the earth at a height of H=180km above the ground. The gravitational acceleration there is g=9.3ｍ·s minus 2 square. Due to the effect of air resistance, in one year Within, the height of the artificial satellite will drop by ΔH = 0.50km. It is known that the resistance F encountered by an object when moving at a speed v in a fluid with density ρ can be expressed as F = 1/2 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 operating at high altitude, the drag coefficient of the satellite can be considered to be C=1, and the maximum cross-sectional area of the satellite is A=6.0m. The radius of the earth is known to be R0=6400km. Use the above data to estimate the atmospheric density where the satellite is located.
This is a final question about 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 can actually be regarded as hitting Wu Bin's gun. Wu Bin can see that the difficulty of this question is to find the correct outstanding elements, ignore the secondary factors, and then make reasonable estimates, 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 question requires estimating the atmospheric density where the satellite is located, it is known from F=12pACv that p is required.
That is to find the relationship between F and v, so it is obvious that this is a problem of functional transformation.
Once you have the ideas to solve the problem, it will be easy, the formula! Just list it!
Solution: Suppose the speed of the satellite before and after 1 year are v1 and v2 respectively. According to the law of universal gravitation and Newton's law, we have
GRＲ1/1 Mm=R1/1 v1, (R1＝R0＋H) (1)
GR2 divided Mm=mR2 divided v2 (R2=R0+H-ΔH) (2)
Once you have the idea and the formulas listed, the rest is just a matter of deduction. However, it involves a lot of mechanical knowledge and the process is not easy.
After finishing the solution, Wu Bin let out a sigh of relief.
‘Damn, there are so many pitfalls. It’s still hard even if you haven’t learned calculus. Otherwise, the force F in the question is a variable force and can be solved using calculus. ’
‘But it might be a trap. Anyway, I can just solve it. ’
After relaxing for a while, Wu Bin looked around and found that many people had left the originally full classroom.
‘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 many experts gathering in the semi-finals. ’
After thinking about it, Wu Bin looked in the direction of Zhang Qiu and found that he was still there, and he kept scratching his head with a pen, looking very uncomfortable.
‘I guess I’ve solved the last question, Amitabha, come on. ’
Originally, Wu Bin thought about handing in the paper, but when he thought that handing in the paper this morning wouldn't put pressure on Zhang Qiu, he thought about it, it was not short of time anyway, and he would try to use other methods to solve the two electrical problems. .