$2\mathrm{.}33.$ Tachometers are a reed$-$type frequency$-$measuring instrument consisting of small cantilever beams with weights attached at the ends. When the frequency of vibration corresponds to the natural frequency of one of the reeds, it will vibrate, thereby indicating the frequency. How large a weight must be placed on the end of a reed made of spring steel $0\mathrm{.}1016$ cm thick, $0\mathrm{.}635$ cm wide, and $8\mathrm{.}890$ cm long for a natural frequency of $20$ cps $?$