2. A stone is dropped from a balloon going up with a
uniform velocity of 5.0 m/s. If the balloon was 50 m high
when the stone was dropped, find its height when the
stone hits the ground. Take g = 10 m/s2
Solution
Given:
Height of balloon when stone was dropped (s)= -50 m/s [since the stone was dropped downwards]
Initial velocity (u) = 5.0 m/s
Acceleration due to gravity (a) =10 m/s2
Using formula,
s = ut + 1/2 * at2
-50 = 5t + 1/2 (-10)t2
-50 = 5t - 5t2
-50 = 5(t - t2)
-50/5 = t - t2
-10 = t - t2
t2- t - 10 = 0
t2 - (3.7 - 2.7) t - 10 = 0
t2 - 3.7 t + 2.7 t - 10= 0
( t - 3.7 ) ( t + 2.7 ) = 0
Either,
t = 3.7
or,
t = - 2.7
Since time ( t ) can never be negative,
t = 3.7 sec
Height of the balloon when stone hits the ground = 50 + ut
= 50 + 5 * 3.7
= 68.5 m