If we have 2 simple pendulums, (one longer than the other) oscillating in SHM in step. The next time they are in step is after 20 seconds has elapsed, during which the time the longer pendulum has completed exactly 10 oscillations. Find the string lengths I found that the length of the long one is 1 metre through the equation: Periodic Time (which is 2) = 2pi X root(l/g) but how do we find the length of the longer one?? thankyou in advance. help very appreciated!