suppose a food packet is dropped from a flying ballon ascending at the rate of 14m/s.then why the initial velocity of the food packet during its fall is also taken to be 14m/s?

inertia!

packet was moving in the system..hence its initial velocity will be same as that of the system

The reason for this is given by the Newtons first law ( Law of inertia).

        Any body which is in state of rest or uniform motion will continue to be in its state of rest or uniform motion untill and unless an external force acts on it.

         In this case of the food packet dropped from a flying ballon ascending at the rate of 14 m/s, at the instant when it is dropped from the ballon no external force has been imparted on it (just dropped). So  we have to take the intial velocity of the food packet as 14 m/s and the direction of the velocity is same as that of the ballon at that instant.

          As the time passes the velocity reduces and reaches zero at the highest point. From that highest point the food packet again falls down, when it reaches the point where it has been left from the ballon it gets the velocity of 14 m/s with direction downwards (This we can get from basic kinematics equations or we can do the energy balance as P.E +K.E =Const).

          So the intial velocity of the food packet during its fall is also taken to be 14 m/s.