Ask your Investigators to time your pendulum for 30 swings back and fourth. They may call out each second just after you call out each swing number.
Swing your pendulum holding the knot or just above the knot with your fingernail. Just as the pendulum gets to one side say "start" for the stop watch Investigators. Then as the pendulum returns to that same position say "1" and some Counters should respond with ...
Now muse with them about how and why the results are what they are.
Look up pendulum motion history and science and let us know what you find out! Time For Science Education: How Teaching The History And Philosophy Of Pendulum Motion Can Contribute To Science Literacy by Michael R. Matthews published in 2000 Kluwer Academic/Plenum Publishers.
As for the math relationship of Time, T in seconds, for a complete back and forth swing equal to two Pi times the square root of the ratio of length, L in centimeters, divided by the gravitational acceleration constant g, about 980 cm per second squared. Another way of stating this relationship is:
For additional pendulum information related to your physical activities consider the link http://www.jug.net/wt/tj8pend.htm to juggling! (or to) "The Story Board(s)" for function festivals.