Help, lost in this question

Question

A rock falls from a tower. As it is falling, its height, h, is given by h = 78.4t – 4.9t² with t in seconds. How many seconds will it take for the rock to hit the ground?

Answer 1

When the rick hits the ground, the height would be 0. So plug zero in for h and then solve.

0 = 78.4t – 4.9t²

This can be solved by the quadratic equation, it also factors.

0 = 4.9t(16-t)

Setting the factors equal to zero and solving

4.9t = 0 and 16-t=0

t=0 and t=16

t=0 is when the rock, LEAVES the ground. The time it hits the ground would be 16 seconds.