Find the Period of Oscillation For your measurements you will only need two items: something to measure length (ruler, meter stick, tape measure, etc.)  something that will function as a stopwatch (the stopwatch setting on your phone's clock, type "stopwatch" into Google, or if you happen to have one, an actual stopwatch, etc.). You will now determine the period of oscillation:  Measure the pendulum length / as the distance from the upper point where the string is tied to the center of mass of the hanging object. For most objects this will be approximately the middle of the object. See left figure on page 2. Swing the mass back a small distance from its resting position. The important thing is that the oscillation distance be small compared to the length of the string, for example 1/5 to 1/10 the length of the string.  Be sure the pendulum swings smoothly and in a plane (i.e., you only want it going back and forth in one direction, not in circles).