Derive Equation 7.4, [ frac{partial}{partial x}left[T(x) frac{partial y}{partial x} ight]+f(x, t)=m(x) frac{partial^{2} y}{partial t^{2}} ] using Newton's second law of motion.

Derive Equation 7.4,

\[
\frac{\partial}{\partial x}\left[T(x) \frac{\partial y}{\partial x}\right]+f(x, t)=m(x) \frac{\partial^{2} y}{\partial t^{2}}
\]

using Newton's second law of motion.