I made this game right, and the gravity in it is quite awesome! People can jump and land and fall and not get stuck or go beneath the ground in any way. I tried to duplicate it but my new games can't, but if you want you can check out my game and if you like the gravity download it and copy the method i used.
Check it out at http://scratch.mit.edu/projects/basilemplis/53020
And if you like trampoline grvaity (bouncing up and down and using awesome gravity skill try http://scratch.mit.edu/projects/basilemplis/53020
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