Question

A balloon starts rising from the ground with an acceleration of 1.25m/s2. After 8sec, a stone is released from the balloon. The stone will-

(a) begin to move down after being released.
(b) reach the ground in 4sec.
(c) cover a distance of 40m in reaching the ground.
(d) have a displacement of 50m.

The answer is (b) but whats wrong in (a)??

Answer 1

The acceleration of the balloon for rising up,a=1.25m/s2

The velocity of balloon at time,t=8s is 'v'.ï½

Usingv=u+at

v=0+(1.25)8

v=10m/s

In 8s the balloon will rise to,

2as=v2-u2

2(1.25)s=102-0

s=40m

Now, balloon will drop down with initial velocity,u'=v=10m/s with acceleration,a'=-g=-10m/s2

and cover distance -40m (as it is moving downwards).

Using,

s=ut+1/2at2

-40=(10)t-1/2(10)t2

We will find,

t=4 or t=-2

Time can not be negative so the required answer is 2 seconds. the stone will hit the ground in 2 seconds.

Answer 2

a is also correct but b is more correct as u know everything falls when released so that's not an answer
hope it helps ...