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

DESCRIPTION: Q: Two identical masses are hung in series from a fixed point alternating with three identical 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.

Click your mouse below to see mpeg videos of these two cases:

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

See Question of the Week #42 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1F20.10) See Demonstration Reference File for several neat articles regarding this demonstration. A 1996 article and the subsequent Addendum to "Which String Breaks?", Phys.Teach. 34, 504 (1996) correct errors in the original papers of 1945 and 1946.

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

SETUP TIME: None.