Ordinary differential equations (ODEs) with variable coefficients appear in nearly every area of physical science and engineering from heat conduction in non-uniform solids to population dynamics in ecology, from beam deflection under distributed loads to electrical circuits with time-varying components. Despite their widespread importance, these equations are often treated in textbooks as isolated algebraic exercises rather than as living tools that connect mathematical formalism to physical understanding. This paper takes a different approach. It starts from the simplest first-order linear ODE and builds systematically toward second-order nonlinear equations with non-constant coefficients, deriving analytical solutions wherever they exist and demonstrating numerical approaches specifically the Runge-Kutta fourth-order method where they do not. Every equation discussed here is chosen because it models a real physical situation, not for abstract completeness. The paper covers five core topics: exact and linear first-order ODEs, the Bernoulli equation, second-order linear ODEs with constant and variable coefficients (including the Cauchy-Euler equation), the power series method for equations with ordinary points, and numerical solution by the RK4 method. For each type, the theory is followed by one or more fully worked examples with step-by-step solutions, and results are presented in tabular form for clarity. The paper concludes with a discussion of how different ODE types appear in specific engineering and physical problems, supported by a comparison table. The goal throughout is mathematical rigour combined with physical intuition showing not just how to solve these equations but why solving them matters.
Jayashri Madhukar Kumbhar (Tue,) studied this question.