A balloon is starting from the ground has been ascending vertically at uniform velocity for 4 sec and a stone let to fall from it reaches the ground in 6 sec. Find the velocity with which the stone hits the ground. ($g= 10 \frac{m}{s^2}$)
The answer given in my book is $42 \frac{m}{s}$ but I got $-60 \frac{m}{s}$ using the equation $v = u + at$. If my answer is wrong please tell why this equation didn't work.
Working :-
$t = 6 s$
$u = 0 \frac{m}{s}$ (left fall means stone is released from rest without any push)
It's under free fall $a = -10\frac{m}{s^2}$
$v = u + at$
$v = 0 + (-10)6$
$v = -60 \frac{m}{s}$