"Gai's exaggerating too much!"
Kitagawa laughed and explained, "I only caught Tsunade-sama off-guard with my speed technique. That's all."
This was the truth.
However, both Jiraiya and Minato had actual combat experience.
They understood perfectly well that enemies encountered during field missions don't have pre-gathered intelligence, and surprise attacks happen frequently.
So even being caught off-guard carried significant weight.
Especially for Jiraiya, who understood Tsunade well.
One careless mistake and he'd get beaten by her!
So Jiraiya looked at Kitagawa with a rather complex expression.
Kitagawa then asked, "Minato, what ninjutsu are you developing?"
By admitting his own techniques first, asking Minato now wouldn't seem presumptuous.
Minato scratched his head somewhat awkwardly. "Well... it's hard to describe at the moment."
"I want to develop a ninjutsu that doesn't require hand signs—concentrating chakra into a highly compressed sphere."
"But I haven't completed it yet."
So it really is the Rasengan!
Kitagawa remembered that developing the Rasengan took Minato three years.
He was fifteen now, but Kitagawa wasn't sure when he started developing it.
Additionally, there was the Flying Thunder God technique. Kitagawa had no idea when Minato mastered that—the original source material never specified.
The equipment I obtained only had super reaction speed as a trait. Hasn't he mastered Flying Thunder God yet?
Or perhaps... he's carrying other equipment elsewhere, since today is a relaxation occasion.
The thought flashed past. Kitagawa pretended to ponder. "A highly compressed chakra sphere?"
"The difficulty in controlling that is extremely high. You'd need form transformation to stabilize the chakra while considering destructive force..."
Minato nodded. "Exactly. I was thinking of making the chakra rotate, but neither the stability nor the destructive force reaches my requirements."
Kitagawa gestured on the water surface before him, creating a vortex as the water rotated. "Something like this rotation, right?"
Minato nodded.
Kitagawa analyzed, "If we compare chakra to water flow, then we use a circular container to hold water, then use our hand to stir it, making the water rotate—whether clockwise or counterclockwise, the water will still expand outward or scatter."
"So returning to a circular chakra sphere held in your hand—clearly, with such rotation, we can't properly contain the rotating chakra that's scattering outward."
Minato's eyes lit up. "Exactly right!"
Jiraiya looked at Kitagawa with a strange expression. This guy... seems to have the same ninjutsu intuition as Minato!
Kitagawa continued, "The attack power of a chakra sphere essentially comes from the rotation... but the dispersive force from rotation not only destabilizes the sphere, it reduces the power."
Minato nodded continuously, silently praising him internally. Such sharp insight!
Gai listened with a completely confused expression, understanding nothing.
Fundamentally, from a physics perspective, centrifugal force, friction, and other factors could be analyzed.
Of course, from a physics standpoint, centrifugal force alone isn't rigorous enough. Using inertial force to describe it more accurately.
The reason rotating chakra scatters is because inertial force drives it outward—insufficient centripetal force.
The so-called centripetal force is the net external force directed toward the center when objects move along circular or curved paths.
Centripetal force is named after the effect produced by this net external force.
This effect can come from elastic force, gravity, friction, or any single force—or the resultant of multiple forces.
If one understood D'Alembert's Principle, the Rasengan would be much easier to comprehend.
To maintain stable circular motion—that is, letting chakra rotate stably in circles—one would need: Active Force + Constraint Force + Fictitious Inertial Force = 0.
So being a ninja requires education too!
However, centripetal force and D'Alembert's Principle were somewhat abstract. Kitagawa needed more intuitive language.
So after thinking for a moment, he said, "I think I understand now. I know what to do!"
What?!
Both Minato and Jiraiya were shocked simultaneously.
That's too fast!
Could this guy be even more of a genius than Minato?
Jiraiya couldn't believe it.
If so, he should have stood out long ago.
After all, Minato and Kitagawa were in the same class.
"Kitagawa, what method did you come up with?"
Minato asked urgently after his shock.
Kitagawa smiled, "Well... it's hard to explain verbally. Let's find a place later and demonstrate it for you!"
Though eager, Minato understood that leaving the hot springs immediately would disrupt everyone's mood, so he quickly calmed down and agreed.
Actually, if not for the premise that Kitagawa developed movement techniques and defeated Tsunade, both Jiraiya and Minato would likely disbelieve or dismiss him.
But now, though incredible, they believed in Kitagawa's ability.
Half an hour later.
After finishing at the hot springs and dressing, everyone came to Minato's training ground.
You could see numerous craters left by unformed Rasengan attempts.
"Kitagawa, can you explain now?"
Kushina was even more eager than Minato.
Previously, Minato developing this technique had caused a misunderstanding between them.
The Rasengan was what Minato developed for her sake. So Kushina hoped for his success more than anyone.
Facing Kushina's urging, Kitagawa smiled and pulled out something he'd bought on the way.
He tied string around a small ball. "Everyone, please focus on the ball."
Kitagawa pinched the string, making the ball move in circular motion via the string. "The ball is rotating. This trajectory of rotation is how chakra should rotate."
"So why doesn't the ball fly away?"
Shizune said, "Teacher Kitagawa, that's simple, right? Because of the string."
Jiraiya basically understood where Kitagawa was going, stroking his chin. "We can't use string or similar things to constrain rotating chakra, right?"
Kitagawa looked at the thoughtful Minato and explained, "What Shizune said about the string isn't wrong, but it's only surface-level."
"Of course, surface-level explanation isn't a problem either. And Jiraiya-sama also isn't wrong."
"Now let me demonstrate more directly."
"Gai, come here. Everyone, please spread out a bit."
Everyone complied.
Kitagawa grabbed Gai's hand, exerted force to swing him in circles, then released him. Gai immediately flew through the air.
But Gai did several aerial flips and landed smoothly without issue.