$A$ block is supported by a cord $A$ from a rigid support,and another cord $B$ is attached to the bottom of the block. If you give a sudden jerk to $B$,it will break. But if you pull on $B$ steadily,$A$ will break. Why?

(N/A) When a sudden jerk is given to the lower cord $B$,the upper portion of the system is not able to receive the force of the jerk in a very short interval of time due to the inertia of the block,and the block tends to remain at rest. As a result,the tension in cord $B$ exceeds its breaking strength,and it breaks.
On pulling the cord $B$ steadily,the force gets sufficient time to be transmitted to the upper cord $A$. In this case,the total tension in cord $A$ becomes the sum of the force applied and the weight of the block $(T_A = F + mg)$,while the tension in cord $B$ is only the force applied $(T_B = F)$. Since the tension in $A$ is greater than in $B$,cord $A$ breaks first.