I was trying to do this problem, but I got t negative, so I went back from the beginning and looked for my errors, but I couldn't find one. Can anyone tell me what I did wrong ? Thanks You drop a ball from a window on an upper floor of a building and it is caught by a friend on the ground when the ball is moving with speed Vf. You now repeat the drop, but you have friend on the street below throw another ball upward at speed Vf exactly at the same time that you drop your ball from the window. The two balls are initially separated by 28.7m. At what time do they pass each other ? Take upwards as positive, yo = 0 will be ground, H will be the distance when two balls pass each other. For the ball going downward: Vf = Vo - gt Vf = - gt -(28.7 - H) = -1/2 gt^2 H = - 1/2gt^2 + 28.7 (1) For the ball going upwards H = Vf *t - 1/2gt^2 H = - gt^2 - 1/2 gt^2 (2) From (1) and (2) -t^2 = 28.7/9.8 --------> negative ?????