Book: # 3

THE PROCEDURE:

The steel ball is suspending from the frame and the metal bar is tied to the string hanging down from the ball.  When a quick jerk downward is applied to one of the two bottom strings with a metal bar, the bottom string breaks. With a gradually increasing downward pull on the other string with the  metal bar will,  the top string will break.

Before any force is applied to the system, the upper string is under a tension equal to the weight of the steel ball, while the lower string is under no tension at all.  When a gradually increasing force is applied to the metal bar, the tension in each part of the system gradually increases.  The tension in the upper string is equal to the weight of the ball plus the downward force applied to the bar.  The tension in the lower string is equal to the force applied to the metal bar.  The upper string will always break first in this case. 

When a quick jerk downward is applied to the metal bar, the steel ball will resist the acceleration according to Newton's Second Law.  The tension in the upper string will increase slightly.  The impulse acts almost exclusively on the lower string.  In this case the lower string will break first.