Conceptual Science

​ Chapter 10-3 is a carefully staged piece of reasoning in which Feynman is less interested in announcing a law than in showing how one earns the right to believe it. The passage reads almost like a laboratory notebook written for the reader, with the air trough playing the role of both experimental apparatus and philosophical device. By invoking Galileo’s struggles with friction and then immediately “fixing” them with modern technology, Feynman situates his argument historically while also making clear that his conclusions rest on controllable, repeatable experiments rather than abstract postulates. The air trough is not decorative: it is what allows motion to persist unspoilt, so that reasoning about collisions can proceed without constantly apologising for real-world imperfections. Methodologically, Feynman begins with the most symmetrical situations imaginable. Equal masses, starting from rest, driven apart by an internal explosion; equal masses approaching one another with equal s...

​ Richard Feynman’s discussion of conservation of momentum in Chapter 10-2 of  The Feynman Lectures on Physics  is a careful construction of a universal physical law from simple but deeply constrained ideas. Rather than presenting momentum conservation as an axiom, Feynman shows how it follows naturally from the mutual character of forces and from the symmetry properties of space and motion. The result is not merely a rule for solving problems, but a statement about what kinds of change are possible in nature. The starting point is the recognition that forces between particles arise through interaction and are inherently reciprocal. When two particles interact, each influences the other, and the changes produced by these influences are linked. The significance of this reciprocity becomes clear when one considers the system as a whole rather than focusing on individual particles. Changes in motion can occur internally, but when all interacting particles are considered together,...

​ In Chapter 10-1 of  The Feynman Lectures on Physics , Feynman situates Newton’s Third Law within a broader methodological discussion concerning the limits of analytical and numerical approaches in classical mechanics. While Newton’s Second Law provides, in principle, a complete prescription for determining motion from known forces, Feynman emphasises that practical and conceptual considerations motivate a deeper examination of the laws themselves. Analytical solutions, where they exist, reveal general structures of motion - parabolic trajectories under uniform acceleration, harmonic oscillations described by trigonometric functions, and elliptical planetary orbits - that numerical methods, though powerful, do not naturally expose. These solutions contribute not merely computational efficiency but conceptual understanding. Feynman then contrasts such analytically tractable systems with those that resist closed-form solutions. Even modest increases in complexity, such as deviations...

​ By the time we reach Chapter 9–7 of  The Feynman Lectures on Physics , the tone of the book has subtly shifted. Feynman is no longer primarily concerned with finding elegant analytical solutions. Instead, he is showing us how physicists actually wrestle complicated systems into submission. The subject is planetary motion, but the deeper lesson is about method, approximation, and the quiet power of Newton’s laws when they are allowed to operate incrementally. Feynman begins by pointing out that techniques which work beautifully for simple systems, such as oscillating springs, do not transfer neatly to planets moving under gravity. The force law is different, and that difference matters. A planet does not experience a restoring force proportional to its displacement. Instead, the force depends on its distance from the Sun in a way that makes the mathematics stubbornly resistant to tidy solutions. Rather than fighting this, Feynman changes strategy. The core idea is to stop thinking...

One of the quiet pleasures of the  Feynman Lectures on Physics  is discovering just how modern many of Feynman’s instincts were. Long before “computational physics” became a standard course title, Feynman was already teaching students how to think like numerical analysts - cautiously, pragmatically, and with a healthy suspicion of algebraic perfection. Chapter 9-6 is a beautiful example of this mindset in action. Here, Feynman sets aside exact solutions and instead rolls up his sleeves to  actually solve  a differential equation step by step, using numbers. The problem itself is simple: a particle subject to a restoring force proportional to displacement, a(t) = -x(t), the equation of motion for a simple harmonic oscillator. But the point of the chapter is not the oscillator - it is the  method . Feynman begins with the most straightforward numerical idea imaginable: break time into small chunks and march forward. He chooses a time step ε  = 0.10 sec, and s...