Question

I want the answer of questions no. 4 &3

Answer 1

4) The potential energy is at its maximum as the velocity is 0 .The kinetic energy will also be 0.

Proof:

Mechanical Energy=Potential Energy +Kinetic Energy

=Potential Energy+0

=Potential Energy

Answer 2

answer no.3= the balloon moves upwards because it is due the Newtons third law of
motion..
According to this law = The force exerted buy balloon on on air is equal and opposite in direction in magnitude the force exerted by air on balloon . the force which balloon exerts on air is called the action and the force exerted by air on Balloon is called reaction .
In simple words an equal and opposite reaction acting on the balloon pushes the balloon in the upwards direction. Due to action of exhaust air from backside , this reaction produces motion in balloon..
I will give you forth answer in comment box latter because iam busy oK
thanks....