Check out this Tips and Tricks video for a solution for creating the feeling of "gravity" in your Scratch project. Adjust the "Y velocity" variable to play around with different levels of gravity -- whether you're on the Earth, or Jupiter!
See video titled Gravity in this Google Drive folder containing CCOW Tips and Tricks cut from Office Hours video. https://drive.google.com/folderview?id=0B6L3TlbW2hcGZERlaUZZODJYZ00&usp=sharing
