Initial velocity u = +19.6 ms-1. The stone after reaching the highest point comes back to the initial point
∴ The displacement = 0.
a = g = – 9.8 ms-2
Let t be the time taken to reach the ground
From s = ut + 1/2at2, we have
0 = 19.6t – 1/2 × 9.8 × t2
4.9t2 – 19.6t = 0
Dividing throughout by 4.9t, we have t = 4s. Hence, the stone reaches the ground after 4s.