A simple pendulum is suspended from a pulley with the help of a long rope. A stop clock is also provided. There are open slots at different heights in the front panel. Bring the bob suspended to the first slot by adjusting the rope length. Pull the bob to the extreme left and release it to oscillate. With the help of stop clock, note down the time taken to complete cycle. For 20 cycles, we can divide the total time noted by 20 to get the time taken for a single cycle. Repeat the same procedure for other slots.

What did you observe? 
When the length of pendulum increases, it oscillates slowly and vice versa.

The science behind it :
Frequency(F) of a pendulum of length ‘L’ = 1/T (Time taken for a single cycle).