A copper rod $($length $=2\mathrm{.}48m,$ radius $=3\mathrm{.}55{10}^{-3}m,$ Young's modulus $1\mathrm{.}1{10}^{11}\frac{N}{m^2})$ hangs down from the ceiling. A $7\mathrm{.}31-kg$ object is attached to the lower end of the rod. The rod acts as a "spring," and the object oscillates vertically with a small amplitude. Ignoring the rod's mass, find the frequency $f$ of the simple harmonic motion.