This document discusses differential equations and related concepts. It defines a differential equation as an equation containing derivatives of dependent variables with respect to independent variables. A first order differential equation contains derivatives of degree one. The document also discusses orthogonal trajectories, solving differential equations for orthogonal curves, and provides an example of finding orthogonal trajectories given stream lines. Additionally, it mentions Newton's second law relating force and momentum, Kirchhoff's voltage law, and initial value problems for differential equations.