1. The problem statement, all variables and given/known data A ball is thrown straight up from the ground with speed v_o . At the same instant, a second ball is dropped from rest from height H - there's no air resistance. Find the time when the two balls collide. 3. The attempt at a solution The path of the ball which is thrown up, B_up, can be described with x_up(t) = ½*(-g)*t^2+v_0*t The path of the ball which is dropped, B_down, can be descriped with H = ½gt^2 Where do I go from here? If i assume that the distance the ball which is thrown up travels is H, then H = ½*(-g)*t^2+v_0*t, and inserted in H = ½gt^2, I get that t = v_0/g. Does this look correct?