An experiment is set up to measure the time that a pendulum takes to swing back and forth 20 times. The pendulum consists of a mass at the end of a string and is released from a certain height above the lowest point in the pendulum's path. For the first part of the experiment, the length of the string is shortened by 5cm and the time is measured. What is the independent variable? What is the dependent variable? What should the controlled variables be?