Answer:
If we wanna measure a pendulum's period, we can use a chronometer watch. But, if we try to measure just one period, we will find too many source of errors, between the reaction time of the human body, and the precision of the watch, it will not be a very accurate measurement. We can reduce those errors, simply by measuring several periods at the same time, and then, dividing the time measured by the numbers of periods we measured.
To obtain betters results, we need to repeat this experiment several times.
Now, if we want to find a relationship between the length of a pendulum and its period, we need to repeat the first experiment for several lengths.
After we have done this, to depicts the results, we can graph the period of the pendulum vs the length, and try to see if there is a clear correlation. Then, we just need to use statistics to find the correlation.
