(i) The two prongs of a tuning fork set each other in resonant vibrations which help to maintain the vibrations for a longer time.
(ii) While crossing a hanging (or rope) bridge, soldiers are asked to break steps because if they are up in steps, all the separate forces being in the same phase, will produce strong energetic vibrations of a particular frequency. If the natural frequency of the bridge becomes equal to the frequency of the steps, the bridge will vibrate will larger amplitude due to resonance and hence the bridge might overturn or crumble down.