A student investigates the factors that affect the terminal speed of a sphere falling through a high-viscosity liquid. Plan an experiment which will enable her to investigate the effect of one factor on the speed at which a glass sphere falls through liquid soap. The apparatus available includes: glass spheres of various diameters, a tall transparent cylinder, a supply of liquid soap, and rubber bands to serve as markers. In your plan, you should: (1) state clearly the factor to be investigated, (2) list any additional apparatus needed, (3) state any key variables to be kept the same, (4) explain how to carry out the investigation, including the measurements to be made and any precautions that must be taken to ensure reliable results, and (5) explain how to use the readings to reach a conclusion.