You can construct a simple pendulum easily by attaching a ball to a string (with tape or a staple) and fixing the other end of the string to a rigid support. (A pencil taped firmly to the end of a desk or table will do nicely.)
a. The frequency of oscillation can be measured by timing the swings. The usual method is to use a watch to measure the time required for ten or more complete swings. The period T (the time required for one swing) is then the total time divided by the number of swings counted and the frequency f is just 1/T.
b. How does the frequency change if you vary the length of your pendulum? (Try at least three different lengths.)