electronic watches keep accurate time using crystal oscillators. inside the watch, there is a tiny block of quartz which vibrates. two opposite faces of the block move alternately toward each other and away from each other. this is a caused by a standing wave in the block. the two opposite faces are at antinodes, and the plane halfway between these two faces is at a node. if the two faces are 7.79 mm apart and the speed of sound in quartz is 3.72 km/s, find the frequency of the vibration.