Spring And Pulley Paradox


To illustrate forces in a potentially paradoxical way.


Spring scale, pulley, and spring gizmo as photographed.


Let your students guess before letting one read the result.




The mass on the hanger in the center pulls down on the spring scale with a force W equal to its weight. Q: If two identical mass hangers are connected over the two pulleys, with a spring and an identical spring scale between the pulleys, what will that spring scale read: 0, W, or 2W? Suggest that it might be zero because the spring "uses up" the force in becoming extended. Alternatively, suggest that it might read 2W because each of the two masses on the ends is pulling with a force W.


Richard M. Sutton, Demonstration Experiments in Physics, Demonstration M-8. Addition of Forces. George W. Ficken, Jr., and Angelo A. Gousios, A "Universal" mechanics demonstration, TPT 13, 169-170 (1975). Anton E. Lawson and Warren T. Wollman, Physics Problems and the Process of Self-Regulation, TPT 13, 470-475 (1975).