Answer
a. If the bob goes back and forth twice each second, the frequency f = 2 cycles/second = 2 hertz = 2 Hz.
b. Period $T = \frac{1}{f} = \frac{1}{2 Hz} = 0.5 s$.
c. The amplitude is the distance from the equilibrium position to the maximum displacement. In other words, this is the distance from the midpoint to the endpoint. The amplitude is one-half of the peak-to-peak distance of 20 cm, which means amplitude = 10 cm.
