A new roller coaster is advertised as having a perfectly efficient (energy is conserved) first drop without the presence of an additional applied force. Upon riding this roller coaster, a group of students question whether the roller coaster is actually perfectly efficient and want to design an experiment to test this question. The students hypothesized that inclined surfaces approach perfect efficiency as the surface increases in steepness. Design an experiment to test the hypothesis stated by the students above. The student has equipment that is typically found in a high school science laboratory. Identify the variable that will be directly manipulated.