Look at the diagram below and answer the following questions:
$(i)$ When a force is applied through the free end of the spring balance $A$,the reading on the spring balance $A$ is $20 \ g \ wt$. What will be the reading shown by the spring balance $B$?
$(ii)$ Write the reason for your answer.
$(b)$ If a balloon is filled with air and its mouth is untied,air is released from its mouth in the downward direction. Write the other observations made by you and justify your answer.

(N/A) $(a) (i)$ Spring balance $B$ will show $20 \ g \ wt$.
$(ii)$ This is due to Newton's third law of motion. When spring balance $A$ exerts a force on balance $B$,balance $B$ exerts an equal and opposite force of reaction on balance $A$. Since the force applied is $20 \ g \ wt$,the reaction force is also $20 \ g \ wt$.
$(b)$ It is observed that the balloon moves in the upward direction. Air rushes out from the balloon in the downward direction (action). As a result,there is an equal and opposite reaction in the form of upward motion of the balloon,which is in accordance with Newton's third law of motion.