WHY DOES IT WAIT?

What will you explore?

Cross and Wheatland, 2012

Changes in tension take time to travel through a spring.

How does it work?

Watch a 90-second cinematic explanation, or explore a story that pauses so you can change its central mechanism. Inspect the source evidence, compare outcomes and resume the narration. This free Lesson Worlds adaptation needs no account or download; younger learners may want help from a parent or teacher.

Inside the cinematic story

1. WHY DOES IT WAIT?

   WHY DOES IT WAIT?

2. A change travels downward

   A change travels downward

3. Two filmed slinkies

   Two filmed slinkies

4. Release the support

   Release the support

   Take control: Where does collapse begin? Keep support held · Release the top

5. Instant or finite collapse?

   Instant or finite collapse?

   Take control: Must turns collapse instantly? Finite collapse · Instant collapse

6. Gravity never switched off

   Gravity never switched off

Discuss the evidence

Before starting, explain what you think this claim means: Changes in tension take time to travel through a spring. After exploring, choose one stage above and describe what its model helps you see. Separate what you observed in the adaptation from what the original researchers measured.

Read the original publication for its methods, evidence and limitations. Ask which part of the explanation the interactive model leaves out, and what additional evidence you would need to test the claim yourself.

Original 3D reconstruction of Cross and Wheatland (2012), Sections III and IV. The scene solves their semi-analytic instant-collapse and finite-collapse models (Equations 10, 14, 17, 24, 25, 33 and 34) with illustrative Figure 4 parameters: 80 turns, mass .2 kg, spring constant .84 N/m, collapsed length .06 m, stretched mass fraction .9, finite-collapse fraction .125 and g=9.81 m/s². Lengths are enlarged five times and time slowed for teaching. The models approximate turn collisions rather than resolving them; the instant model has an unphysical initial top velocity. After collapse, the packed spring follows center-of-mass free fall without rebound or floor impact. Colored wavefront and center-of-mass markers are explanatory. This is not experimental footage, digitized data, a general collision simulation or an exact fit to either tested slinky. The source tested two slinkies and found qualitative agreement; suspension method affected initial motion and no detailed error analysis was attempted. Bottom suspension does not cancel gravity; the whole spring’s center of mass falls.

Original research

Modeling a falling slinky

R. C. Cross and M. S. Wheatland · American Journal of Physics 80, 1051–1060 (December 2012) · 2012

Educational adaptation by Lesson Worlds. Not affiliated with the authors, the University of Sydney or the American Association of Physics Teachers. The original publication may have its own access terms.