In the students' investigation, the dependent variable is the motion of the block. This is because the motion (which can be described by displacement, velocity, or acceleration) is what the students are measuring and what changes in response to the different levels of force applied.
The independent variable is the amount of force they apply to the block. This is the variable that the students are actively changing during the trials to see how it affects the block's motion. Since they are conducting an experiment to understand the relationship between force and motion, the force applied is the variable they control, and the motion of the block is the variable they observe. The mass of the block and the near absence of friction are controlled variables, kept constant to ensure a fair test of the relationship between force and motion.
