PURPOSE: To demonstrate inertia of rest in a counterintuitive way.

DESCRIPTION: Q: Two masses are hung in series from a fixed point alternating with three strings as photographed. When you pull downward on the third (bottom) string, which of the strings will break: the top, the middle, or the bottom string? A: It depends on how you pull. If you pull very quickly, the bottom string will break, due to the inertia of the bottom mass. If you pull slowly, the top string will break, because the weights increase the tension in the top string.

SUGGESTIONS: Ask your students to vote before doing the experiment; then break the string with the fewest votes.

REFERENCES: See Demonstration Reference File for several neat articles regarding this demonstration.

EQUIPMENT: Stand with two weights, three strings, and a pulling bar (hanging from the lowest string), as photographed.

SETUP TIME: None.

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