The so-called "Angular Momentum Fist" was actually just a joke made by Cheng Ying. Back when he was trying to teach advanced physics to Dugu Bo, it was clear the latter didn't understand a word of it. So Cheng Ying used a martial analogy to explain:
For example, if someone punches you and you spin with the force to neutralize the impact, that's an application of the law of angular momentum. Then, borrowing from the Taijiquan principles of his past life, Cheng Ying explained a few moves and shamelessly named the technique the "Angular Momentum Fist."
It had been a joke at the time who would have thought Dugu Bo had taken it seriously? Not only that, he had gone and developed his own version of the technique. Whether his move would work or not, this kind of clash would leave him seriously injured no matter what.
At this point, Dugu Bo could have conceded with dignity. Even if he surrendered now, Tang Chen would respect his strength. But clearly, Dugu Bo was determined to prove himself.
As Cheng Ying and Bingdi rushed forward, Dugu Bo's palms met Tang Chen's Haotian Hammer. The terrifying force made the bones in his arms creak under the pressure.
Still, he gritted his teeth through the pain, one foot rotating like a compass point, spinning himself like a top. He twisted away from the hammer's direct blow and, under the momentum of the spin, struck Tang Chen's chest with a palm.
The force he'd absorbed from the hammer was returned in full to Tang Chen's chest. No one knew better than Tang Chen himself just how powerful his own attack was.
The heavy impact sent him flying backward, his grip on the hammer lost, carving a trench across the ground as he landed.
Dugu Bo didn't come out unscathed either. Force works both ways—he too was thrown through the air. Thankfully, Cheng Ying caught him in time. Even so, Dugu Bo was in bad shape: both arms fractured, blood seeping from his mouth and nose.
"Teacher… I won," Dugu Bo forced out before finally collapsing unconscious.
Cheng Ying looked at Tang Chen—who was still alive and kicking, albeit dangling by the scruff of his neck in Bingdi's grasp—and decided not to tell Dugu Bo the real outcome.
Just then, Tang Chen, hanging from Bingdi's hand, looked at Dugu Bo with genuine admiration and said, "I lost. Sister Bingdi caught me just in time and cleared the poison around me. Otherwise, I would've landed straight in that toxic mist and lost for sure. He's the first peer who's ever beaten me."
Hearing this, Cheng Ying glanced down at the unconscious Dugu Bo with a new level of appreciation. To have calculated things that far ahead just for the sake of winning… No wonder, in the original story, he ranked so low among the Title Douluo—it must've been a bitter pill for someone like him to swallow.
As Cheng Ying reflected, Tang Chen had already slipped out of Bingdi's grip, knelt before him, and bowed in the traditional disciple's salute. "Master, please accept me as your student! Please teach me the last move my senior used!"
Cheng Ying accepted the salute and helped him up. "What Dugu Bo used just now wasn't a soul skill I taught him—it was something he comprehended himself from the physics I passed on to him. If you wish to learn it, I'll have to start by teaching you physics."
Tang Chen nodded, glancing toward Dugu Bo, now being carried off on a stretcher by medical staff in white lab coats. "Please teach me, Master."
On that note, it's worth mentioning that while Cheng Ying wasn't a doctor, he had enough medical knowledge to establish a decent hospital in Heaven Dou City. After hiring and training some local physicians, the facility had become quite respectable. Most patients were soul masters injured in combat or beast hunts, so Dugu Bo's bone fractures were no problem for the staff.
Lacking the formula for Ma Fei San (ancient Chinese anesthetic), Cheng Ying could only provide ether synthesized in his lab as anesthesia. While most ordinary soul masters couldn't afford it, his own disciples naturally got the VIP treatment.
Once Dugu Bo was settled at the hospital, Cheng Ying took out pen and paper and began teaching Tang Chen physics.
"To start with, you need to understand something fundamental: does motion require force to continue?"
This was Cheng Ying's way of addressing the infamous academic trap left by Aristotle. His first counterintuitive conclusion—that motion does not require continuous force—stunned Tang Chen, who couldn't find any way to argue against it.
Then came the differences between mass and weight, speed and acceleration… and experiments such as: if two iron balls of different masses fall from the same height, which one hits the ground first?
The explanations, accompanied by hands-on demonstrations, left Tang Chen dumbfounded. But the more he immersed himself in this framework, the more he saw the logic behind it. The realization that ideas humanity had taken for granted for millennia were, in fact, incorrect, filled him with awe at his teacher's vast knowledge.
Cheng Ying lectured the entire afternoon, until his throat ran dry—only barely managing to explain the concept of mass. Even so, Tang Chen remained wide-eyed, hungry for more.
"Teacher," he asked, "everything you've taught me today is unchangeable truth about the world. But what do these things have to do with the ideal society you mentioned before? A world where farmers have land, everyone has a home, the elderly are cared for, and children are educated?"
Cheng Ying smiled. "All of this is the foundation of a new era. But before I give permission, you must not share what you've seen or heard today with anyone."
Saying that, Cheng Ying led Tang Chen to the backyard.
Every afternoon, Bingdi would spend a little time at the shooting range playing target games. Just as they arrived, Tang Chen saw her dressed in a midriff-baring shirt and shorts, holding two silver metal objects he didn't recognize. She performed a mid-air spin…
Bang bang bang bang— Twelve shots rang out in rapid succession, sounding like firecrackers popular in Heaven Dou City recently.
At the same moment, the twelve targets surrounding her were each pierced dead center—in the bullseye.
Landing smoothly, Bingdi spun both revolvers in her hands and holstered them at her waist with a flourish.
"Well? Was that cool or what? That last move—ring barrage—wasn't it totally awesome?" she asked, reverting from a stylish western gunslinger back to her usual cutesy self the moment the guns were holstered.
"Yeah, super cool. It'd be even cooler if you didn't use up all my training ammo," Cheng Ying sighed as he lifted her onto the windowsill.
Her little feet swung innocently as she blinked those big watery eyes at Cheng Ying, as if asking: Which is more important, the bullets or my happiness?
Cheng Ying sighed helplessly, pulled out a revolver from his belt, and began demonstrating to Tang Chen. "I told you before—two points define a straight line. So when aiming, you align the target, the front sight, and the rear sight into one straight line."
Holding the gun in a textbook two-handed grip, he aimed at a distant target and gently pulled the trigger.