booktalesy Chapter 56 This university physics is a bit too easy, isn't it?
Chapter 56 This university physics is a bit too easy, isn't it?
The lecture hall can seat 300 people. University physics is a general education course, and students from the mechanical, electrical, and automation majors are crammed into one class.
Standing on the platform was Zheng Tiancheng, who was in his early forties. He had a buzz cut, wore a black jacket, dark gray casual pants, and leather shoes. He looked lean and capable.
He opened the roll call and started reading the first name.
"Li Dong".
"arrive."
"Sun Yaqi".
"arrive."
When I got to the third row, I sped up.
Zheng Tiancheng has a habit of not looking at people when he pronounces their names, but only listening to their voices.
You can tell pretty much who's there and who's not just by listening.
"Lufeng".
In the back row of the classroom, Zhang Wei was burying his face behind his textbook, frantically sending messages to Lu Feng.
Upon hearing the name, he nearly dropped his phone.
There was a moment of silence.
Zhang Wei gritted his teeth, forced himself to lower his voice by half a degree, and shouted.
"arrive!"
Zheng Tiancheng's pen hovered over the roster for a moment, then didn't fall.
He looked up, his gaze sweeping over the first few rows before settling on the area behind.
"You're from Lu Feng?"
"Yes...yes."
Zheng Tiancheng pushed up his old-fashioned metal-framed glasses and twitched the corner of his mouth to one side.
Do you think I'm blind?
Zhang Wei's smile cracked.
Zheng Tiancheng threw the roster onto the podium, put his hands in his pockets, and leaned back slightly.
"I went to watch the final of the Mechanical Innovation Competition last month. I was in the second row of the judges' panel and didn't leave the whole time. I could see that young man in a silver-gray mecha suit standing in the center of the stage very clearly."
He pointed in Zhang Wei's direction.
"He doesn't look like you at all."
A burst of suppressed laughter erupted from below.
Zhang Wei froze in his seat, his face as red as a crab just pulled from a steamer.
He turned his head and looked around. All the classmates around him were laughing, and several girls covered their mouths, their shoulders shaking uncontrollably.
"Old Lu, it's not that I don't want to help you, but you've suffered a famous loss..." Zhang Wei wailed at his phone screen.
I used to just shout "Here!" and get away with it, but who knew this teacher had actually seen the real person in person.
Just when I was so embarrassed that I wanted to crawl into my desk drawer, the back door of the classroom made a soft sound.
Everyone's eyes turned to that direction at once.
Lu Feng pushed open the door and walked in.
A white shirt, sleeves rolled up to mid-forearm, and a backpack slung over one shoulder.
A thin layer of sweat clung to his forehead, clearly indicating he had run there.
He glanced around the classroom and quickly spotted Zheng Tiancheng on the podium.
Zheng Tiancheng stared at the face at the door, and the corners of his mouth slowly curled up.
"It must be Lu Feng himself."
"Yes, hello teacher."
Zheng Tiancheng turned his head to look at Zhang Wei, who was still standing in the back row, and then looked at Lu Feng, his tone playful.
"Just now, a six-eared macaque impersonated you, Sun Wukong, and almost succeeded."
Laughter erupted in the classroom again, this time even more unrestrained than before.
Zhang Wei wished he could fold himself into an A4 sheet of paper and stuff himself into his textbook.
Lu Feng looked towards the back row following the sound of laughter.
Zhang Wei stood there, his face flushed and his neck thick, his eyes filled with the message, "Brother, I'm doing this for your own good."
Loyalty and righteousness.
Lu Feng made a mental note of it.
Although the methods were a bit foolish, he acknowledged the favor shown to him for stepping up in a crucial moment.
"Lu Feng, classmate." Zheng Tiancheng put away his smile and his tone returned to the formality of a classroom.
He pulled a whiteboard marker from the podium and twirled it in his hand.
"You were late. And your classmate answered the bell for you. According to classroom rules, this needs to be dealt with."
Zhang Wei shrank his head even further in the back row.
"but."
Zheng Tiancheng changed the subject, a light that was extremely familiar to Lu Feng surfacing in his eyes.
He had seen that kind of light in Li Hongde's eyes before.
"I've long heard that Lu Feng has extraordinary expertise in mechanics. Since we've met today, how about this?"
He removed the pen cap, walked to the whiteboard, and turned around.
"I'll give you a question. If you can answer it, this matter between you and your Six-Eared Macaque classmate will be wiped clean."
The atmosphere in the classroom changed instantly.
All three hundred people sat up straight in unison.
The boys who had been whispering to each other stopped what they were doing, and a girl in the front row who was touching up her makeup forgot her lipstick on her lips.
"A physics teacher is testing a mecha expert in class? This is hilarious!"
"Isn't Lu Feng from the mechanics department? University physics covers a very broad range; what if they ask a thermodynamics question..."
"Didn't you see? Teacher Zheng's eyes were clearly trying to fish for us!"
Whispers emerged from every corner.
Lu Feng looked at Zheng Tiancheng.
Zheng Tiancheng was also looking at him.
Their eyes met briefly in mid-air.
Lu Feng understood.
The look in Teacher Zheng's eyes was exactly the same as when Li Hongde drew that gear transmission structure problem on the blackboard.
It's not a challenge, it's a test.
Or more accurately, it was a test.
"No problem, teacher, please give me a question."
Lu Feng took two steps forward and stopped next to the first row of desks.
Zheng Tiancheng turned around and quickly drew a picture on the whiteboard.
A uniform thin rod is fixed at one end to a hinge support, and a small ball of mass m is suspended at the other end.
The rod has a mass of M and a length of L. Next to it are labeled the moment of inertia, angular velocity, and a very inconspicuous restoring torque.
"This is a compound pendulum system with elastic hinges."
Zheng Tiancheng put down his pen and turned around.
"Please derive the expression for the natural frequency of this system under small amplitude vibrations."
"Note that the mass distribution of the rod itself should not be ignored."
The classroom fell silent.
Several first-year students with some basic knowledge in the first two rows frowned.
This question is not easy.
The calculation of rotational inertia and the modeling of elastic restoring torque also require linearization processing under the micro-amplitude approximation.
This isn't something from a first-year physics textbook; it's closer to the realm of theoretical mechanics.
Zhang Wei muttered quietly from the back row, "Is this a university physics question? It looks more like a graduate entrance exam..."
Zheng Tiancheng put the pen cap back on and crossed his arms over his chest.
"Is ten minutes enough?"
Lu Feng did not answer the question.
He walked up to the whiteboard and took the pen from Zheng Tiancheng.
There was no draft paper, no pauses.
The pen tip landed directly on the whiteboard.
Let the moment of inertia of the rod about the hinge point be I_rod = ML²/3.
"The moment of inertia of point mass m at the end of the rod is I_m = mL²."
"The total rotational inertia of the system is I = (M/3 + m)L²".
His handwriting is small, but the structure is clear, with just the right amount of spacing between each step of the derivation.
"The elastic restoring torque is -kθ, and the equivalent gravitational restoring torque is -(Mg·L/2+mg·L)sinθ."
"Under the slight amplitude approximation, sinθ≈θ".
The pen tip showed no hesitation whatsoever.
"Equation of motion: Iθ̈=-(k+MgL/2+mgL)θ."
"The natural angular frequency ω = √[(k + MgL/2 + mgL)/((M/3 + m)L²)]."
Finish with the last square root sign, cap the pen, and put it back in the chalk trough.
From picking up the pen to putting it down, it took forty seconds.
Zheng Tiancheng stared at the lines of derivation on the whiteboard, his pupils visibly contracting.
No one spoke in the classroom.
The breathing of three hundred people could be heard clearly.
Zheng Tiancheng took a step forward and pointed to the line of motion equations.
"Regarding the equivalent gravitational moment, you placed the center of gravity of the rod at L/2. Did you consider the possibility of uneven mass distribution in the rod?"