Question

A piece of stone is thrown vertically upwards. It reaches the maximum height in 3 second. If the acceleration of the stone be 9.8 m/s directed towards the ground, calculated the initial velocity of the stone with which it is thrown upwards.

Answer 1

at the max . height the final velocity of stone becomes 0 and t=3 and a=-9.8 because upward motion
v=u+a×t
put values
o=u-g×t
u=g×t
u=9.8×3=29.4m/s

Answer 2

"The time to reach the maximum height is 3 seconds. If the acceleration of stone 9.8 m/s directed towards the ground, then the initial velocity becomes 9.8×3=29.4m/s.  We need to consider at the maximum height final velocity of stone turns into 0 and t=3 and a=-9.8 as upward motion

v=u+a×t

once we apply the values

o=u-g×t

u=g×t

u=9.8×3=29.4m/s

"