A student takes granite, a sample of igneous rock, and masses it. She records the initial mass. She then places the rock into a rock tumbler, which mechanically tumbles the rock on a continuous cycle. After two weeks, the student removes the rock from the tumbler
and takes its final mass. She makes observations that the rock seems smoother and has lost 8% of its mass over time.
Select the independent variable in this experiment and the step of the rock cycle the student is most likely modeling.