For my science fair project, I am testing the effect of changes in mass on the time it takes a car to complete a roller coaster track. The track is made of hotwheels track and has an initial ramp, two turns and a loop. I used the same car both times (once without added weight and once with an additional 10 grams). After testing for many trials, I found that the lighter car finished the track faster than the heavier car. This is the opposite of what I thought would happen. I thought that because the heavier car had greater potential energy, it would go faster. As I research this question, I start the think that the two cars should have finished at the same time regardless of mass. What is going on?