Exploring Science: Help with Pendulum lab? ?

To create and experiment for science, we had to use washers, a string, a pole, and a stopwatch. We had to try and figure out a way to change the length of the pendulum's period (from when it leaves one spot until it returns). We added one extra washer each time and timed it: shorter. I understand the momentum behind it, but is there any law of gravity to prove it?