When a tuning fork is set into vibrations, its two prongs vibrate in opposite phases. One prong of the fork is forced to vibrate by the other and vice-versa. Hence by holding its stem in the hand, a tuning fork can be set into vibrations and no external force is required to maintain its vibrations.
If one of the prongs is cut-, the vibrations of the tuning fork will soon out and can be maintained only by some external periodic force.
