A ball is thrown upward with speed 12 m/s from the top of a building. How much later must a second ball be dropped from the same starting point if it is to hit the ground at the same time as the first ball? The initial position is 24 m above the ground. thanx in advance

Question

A ball is thrown upward with speed 12 m/s from the top of a building. How much later must a second ball be dropped from the same starting point if it is to hit the ground at the same time as the first ball? The initial position is 24 m above the ground.  thanx in advance

anonymous · Answer

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nw we should calculate the time for first ball to reach the earth. 
s=ut+1/2at^2
24=-12t+(10/2)t^2
5t^2-12t-24=0
nw solve this to get the time