having trouble setting this up and isolating the key information to solve this problem. here it is. question: A ball is thrown straight up from the edge of the roof of a building that is 40.0 m high, and a second ball is dropped from the roof 2.00 s later. What is the initial velocity of the first ball if both balls hit the ground at the same time? answer: 15.6 m/s ======================= my effort: alright i started by trying to apply these formulas.. v0 = v + at .... x = x0 + v0t + 1/2at^2 --> tried using this one. .... v^2 = v0 + 2a(change x) i think i'm not really using x in the equations so i'll change them to y because were doing verticle drops here. ... also, i must be forgetting steps here. i thought that the middle formula would work best, so i plugged in what i knew and used gravity (9.80) what we know: x = 0 (because x final hits the ground) x0 = 40.0m because thats where both balls are when there first released. v0 = thats the variable were trying to find t = 2seconds ? NOT SURE if we can use 2 seconds here. a = i used gravity for this variable (9.80) i'm not sure why we use - or + though, maybe a quick explanation here please. .... okay. so as i said, i used the 2nd equation: x = x0 + v0t + 1/2at^2 i plugged the answers in: and calculated: 10.2 m/s i think i'm missing a big step here. please help. thanks.