On June 21st, "Dr. Charles Zhang's Physics Course" once again hosted a special Gaokao problem-solving session. Charles Zhang, founder, chairman, and CEO of Sohu.com (NASDAQ: SOHU) and a Ph.D. in physics, live-streamed his analysis of two highly challenging final physics problems from the 2026 Gaokao: Question 16 from the Jiangsu paper and Question 13 from the Shandong paper.
This marks the third consecutive year the course has featured this dedicated Gaokao session. This year, the event was attended by renowned physics professors from institutions including Tsinghua University, the Hong Kong University of Science and Technology, Ningbo University, and the China Aerospace Science and Industry Corporation's Second Academy. Also participating were popular physics science communicators from the Sohu Video platform and nearly 70 high school students from top schools like Hengshui High School, the High School Affiliated to Renmin University of China, and the High School Affiliated to Beihang University.
The Core of Physics: Understanding the Process
During the livestream, Zhang not only solved the two complex problems but also engaged in in-depth discussions with students and teachers on cultivating physics thinking and self-directed learning methods. He emphasized that learning physics requires active thinking and personal derivation, not passive knowledge reception.
Zhang noted that analyzing Gaokao physics problems post-exam has become a tradition for his course. This year, he deliberately selected the extremely challenging Jiangsu Question 16 and Shandong Question 13.
In tackling the Jiangsu problem involving a collision of spring-connected masses, Zhang demonstrated classic physics thinking: simplifying complexity and analyzing the system as a whole before its parts. The problem involved three balls connected by springs on a smooth surface. For the first part on one-dimensional elastic collision, Zhang pointed out the spring is undeformed at the instant of collision, allowing a quick solution using momentum and kinetic energy conservation for the large and middle balls. For finding maximum elastic potential energy, he explained it occurs when all balls share the same velocity, converting maximum kinetic energy. The hardest third part required finding the mass ratio for a second collision when the spring returns to its natural length. Instead of complex trajectory integrals, Zhang shifted perspective to the system's center of mass, showing the condition is met when the large ball's post-collision velocity equals the system's center-of-mass velocity, elegantly solving it with a simple formula.
He advised the audience to understand motion from the center-of-mass perspective, revealing that the "second collision condition" is equivalent to the large ball landing at the system's center of mass post-collision. Zhang stated the real difficulty in physics lies not in memorizing rules, but in translating a problem's description into an accurate physical process.
The subsequent Shandong problem, concerning charged particle motion in multiple magnetic field regions, similarly showcased Zhang's precise grasp of physical processes. The problem had three regions with different magnetic field strengths. Zhang first calculated circular motion radii using the Lorentz force law, then determined a single particle's trajectory through step-by-step analysis. For the third part, he formulated algebraic equations from prior insights, establishing spatiotemporal matching conditions for two chasing particles and finding the first meeting time by identifying integer solutions.
Encouragement and Learning Strategies
During the interactive segment, a student from Hengshui High School shared a graphical-numerical approach for the Shandong problem's third part, visualizing particle motions as periodic polygonal lines on a displacement-time graph and identifying a condition for intersection. On-site experts acknowledged the clever intuition behind this method for saving time on fill-in-the-blank questions, while noting it required similar verification steps.
When discussing the elastic collision problem, a student from Beihang University Affiliated High School asked about developing "center-of-mass thinking." Zhang provided intuitive guidance: the large ball hits an entire spring-connected system, not an isolated ball. Post-collision, the system moves uniformly with its center-of-mass velocity; for a second collision, the large ball's velocity must relate to this. By imagining extremes like "flying away and not catching up" or "sticking together," one naturally arrives at the center of mass as the key. He further elevated the concept by analogizing it to the classic "two-body problem" in astrophysics, where introducing reduced mass simplifies complex dual-body motion to single-body motion.
Another Hengshui student extended their thinking on the Jiangsu problem, attempting to derive the trajectory equation for a ball constrained by a spring and finding it was not simple harmonic motion due to a nonlinear restoring force. An expert added that the solution involves elliptic functions beyond high school scope. Zhang used this as an example, stating, "Physics problems are interesting; one problem can lead to much deeper places."
Facing these demanding final questions, Zhang repeatedly comforted the high school students: "It's impossible to solve these in the short exam time. It's okay if you can't solve them; don't feel discouraged. Doing the simpler ones well is still enough for university." He noted that the Gaokao tests not just knowledge of conservation laws but also the accurate description and understanding of physical processes. He advised students to develop exam skills, learning to think from the problem-setter's perspective to discern what is being tested and save time.
Mastering Principles Through Derivation
On how to learn physics well, Zhang stated: "The most important thing when facing a physics problem is to first thoroughly ponder the descriptive language, figure out what it really aims to test, and what direction the parameters given by the problem-setter point towards. You know the physical laws; the difficulty is understanding the problem."
Zhang also recommended the "Feynman Technique," dividing study time into thirds: one for reading and absorbing knowledge, one for mentally visualizing the physical scenario, and one for personally deriving solutions. "Thinking and derivation use different brain regions. Thinking doesn't require writing, but your mind must be active, pondering fundamental questions and the whole picture. Derivation requires high concentration and is also very efficient."
He opposed passive learning and rote memorization of formulas, emphasizing that the process of derivation with focused concentration is key to truly appreciating the beauty of fundamental physics principles, rather than relying solely on AI tools like GPT for answers. On-site physics experts also provided multidimensional learning advice, combining the cultivation of experimental physics intuition with frontier research methods like using calculus to solve complex problems in gravitational waves and black holes in astrophysics.
This is the third consecutive Gaokao session for "Dr. Charles Zhang's Physics Course." Over the years, Zhang has not only delved into the core concepts of various question types but also systematically outlined solution logic, summarizing universal and efficient problem-solving approaches. In the 2024 session, he used geometric methods for Shandong's charged particle question and periodic patterns to simplify Hunan's multi-ball collision problem. In the 2025 session, he employed center-of-mass motion analysis for two-body problems, drawing analogies to celestial mechanics and using Kepler's laws to avoid redundant calculations for electromagnetic field ellipse motion questions.
After nearly five years of dedicated science communication, "Dr. Charles Zhang's Physics Course" has accumulated over 280 live episodes, with its series of popular science books being collected by the National Library of China. The livestream content covers an extremely wide range, from frontier theories like general relativity and quantum mechanics to analyzing the scientific principles behind everyday phenomena. The course has also visited campuses including Tsinghua University, Chongqing University, and the Hong Kong University of Science and Technology for interactive exchanges with physics students.
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