[填空题]
Waves travel through a wave tank. A small bob on the surface of the water is attached to an elastic rope which is fixed to the base of the tank.
The graph below shows the variation of vertical displacement of the bob with time.
1.Use the figure and the graph above to determine:
（1）the frequency of the water wave. f = Hz
（2）the speed of the water wave. v = $m\,s^{-1}$
2.State two times when the bob is at a crest of the water wave and two times when the bob is at the trough of the water wave.
