Inertia - Masses Hanging In Series

Purpose

To demonstrate inertia of rest in a counterintuitive way.

Equipment

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

Suggestions

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

Images

c3-03

Description

: 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.

References

Sutton, Demonstration Experiments in Physics, Demonstrations M-100. Inertial Reaction and M-101. Breaking a Rope by Sutton, Demonstration Experiments in Physics, Demonstrations M-100. Inertial Reaction and M-101. Breaking a Rope by Inertia. ◙P. LeCorbeiller, A Classical Experiment Illustrating the Notion of "Jerk," AJP 14, 64-65 (1946). ◙Frank G. Karioris, Inertia demonstration revisited, AJP 46, 710-713 (1978). ◙Steven H. Schot, Jerk: The time rate of change of acceleration, AJP 46, 1090-1094 (1978). ◙Stephen Luzader, Letter: What a Jerk!, TPT 26, 423 (1988). ◙T. R. Sandin, The Jerk, TPT 28,