Please, I am not sure about this problem. It reads this way:
Two identical pendulums X and Y each consist of a small metal sphere attached to a thread of a certain length. Each pendulum makes 20 complete cycles of oscillation in 16s. State the phase difference in radians, between them motion of X and that of Y if
(a) X reaches maximum displacement at the same time as Y reaches maximum displacement in the opposite direction.
I know the phase difference is calculated by
Phase difference =360 X Change in t/Time period. (Or 2pi in radians)
I am able to establish the time period as 0.8s. (16/20). So, for the equilibrium positions to maximum displacement (Both negative and positive directions ) it will take 0.8s. But I got stuck from here. I am not sure how to calculate the change in time. I am tempted to say 0.8s. But that seems to imply the phase difference is zero.