  This work focuses on a theoretical analysis of a well-known inertia
demonstration, which uses a weight suspended by a string with an extra string
that hangs below the weight. The point in question is which string, the upper
or lower, will break when pulling down on the bottom edge of the lower string
at a constant pulling speed. An analytic solution for the equation of motion
allows us to identify the critical value of the pulling speed, beyond which the
string breaking varies from one to the other. The analysis also provides us a
phase diagram that illustrates the interplay between the pulling speed and the
string's elasticity in the pull-or-jerk experiment.
