1. The problem statement, all variables and given/known data An Astronaugt is doing an experiment on an alien planet. She fires a ball straight up towards a target ball that is dropped at the same time as the lower ball is launched. If air resistence is ignored, how long does it take for the two balls to collide? 2. Relevant equations Kinematics equations. 3. The attempt at a solution So we know that the two balls have three things in common. Time of collision, acceleration, and the displacement that they'll collide. d1=.5at^2 and d1=.5at^2 + v1t (for the ball being launched) I subed the two equations into eachother and it didn't really work out... I'm thinking it has something to do with the signs or are my equations wrong all together? Thanks for the help.