Answer:
[tex]v= - 68.6\ m/s[/tex]
Step-by-step explanation:
The velocity of an object moving with a constant acceleration rate is given by:
[tex]v(t) = v_0 +a*t[/tex].
In this case, assume that the initial velocity is zero, acceleration is only due to gravity and that the time of fall just before hitting the ground is 7 seconds:
[tex]v(7) = 0 -9.8*7\\v= - 68.6\ m/s[/tex]
The velocity of the balloon when it is about to hit the ground is -68.6 m/s.