Suppose you wanted to do an experiment to find out if changing the length of a pendulum changed the amount of time it takes the pendulum to swing back and forth. Which pendulums from the above diagram would you use for the experiment?
If you started out knowing nothing about pendulums, then you'd want to use #1 and #3 for an experiment with meaningful results.
Why ?
Thinking clearly, you'd look at a swinging pendulum, and realize that there are only two things about it that might affect its period: ==> its length, and ==> the weight on the end of it.
Since this first experiment focuses on the length, you want to keep everything else constant. So you'd want to compare pendulums with different lengths but the same weight.
Later on, you'll do the other experiment. You'll compare pendulums with the same length but different weights.
