The Floating Slinky Drop
A dropped slinky reveals that force and motion information travel through a medium as a wave, not instantly — so the bottom of the spring only 'learns' it has been released once the wave arrives.
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Start when you are ready to enter this Stage's 8 scenes and explore, respond, and learn as you go.
When you release a hanging slinky from its top, why does the bottom hang motionless in mid-air for a moment instead of falling immediately?
Hold a slinky by its top, let it hang, then drop the whole thing — the bottom of the slinky defies gravity and hovers in mid-air for a fraction of a second before falling.
Everything you drop should fall together. So why does the bottom of the slinky seem to float while the top plunges downward, only catching up later?
Slow-motion video of a dropped slinky, a side-by-side comparison of the top and bottom positions over time, and a simple simulation that lets the learner tug on a coil and watch what travels downward through the spring.
The information about the release travels down the slinky as a wave, and until that wave reaches the bottom coil, the bottom has no idea it has been dropped — so it keeps doing exactly what it was already doing: hanging.
If I let go of the top, the whole slinky should fall together as one object, like dropping a rope, because every part is connected to the part above it.
- Full mathematical derivation of wave speed in a spring
- Detailed treatment of longitudinal vs. transverse waves beyond what the slinky needs
- Air resistance effects on the later fall
- 01A Slinky That Defies GravityslideQuestion
Introduce the setup: hold a stretched slinky by its top, let the bottom hang freely in the air, then drop the top. Pose the driving question about why the bottom hovers for a moment before falling.
- Setup: hanging slinky released from the top
- Observation: bottom appears to float in mid-air
- Driving question: why doesn't the bottom fall immediately?
- 02Predict the Bottom's MotioninteractivePrediction
Show a simple diagram of a hanging slinky about to be released at the top. Ask the learner to predict what happens to the bottom in the first fraction of a second after release.
- Pick one prediction: bottom falls immediately, bottom hangs briefly then falls, or bottom rises first
- Commit before seeing the evidence
- Notice the intuition that connected things should move together
- 03Tracking Top vs. BottomslideEvidence
Present slow-motion footage and a position-vs-time comparison graph of the top coil and the bottom coil after release, showing that the bottom stays nearly stationary while the top accelerates downward.
- Top coil moves downward immediately upon release
- Bottom coil remains at almost the same height for a noticeable delay
- Both eventually fall together once the collapse reaches the bottom
- 04Send a Pulse Down the SlinkyinteractiveEvidence
Let the learner pluck or compress a coil at the top of a simulated hanging spring and watch a disturbance travel downward coil by coil, leaving the lower coils undisturbed until the pulse arrives.
- Disturbance at the top does not instantly change the bottom
- The change propagates as a traveling wave through the coils
- Each coil only 'knows' about the disturbance when the pulse reaches it
- 05A Wave of 'Release' Traveling DownwardslideExplanation
Explain that releasing the top removes the upward support there, and that change is communicated to lower coils as a mechanical wave of changing tension. Until the wave arrives, the bottom still experiences the same tension from the coils above, so Newton's first law keeps it hanging in place.
- Removing support at the top changes the tension locally first
- That tension change travels downward as a wave, not instantaneously
- Bottom keeps its current state — hanging — until the wave reaches it
- 06What If the Slinky Were a Rigid Chain?slideBoundary
Contrast the slinky with a chain or a stiff rod: if the connections transmitted force instantly, the bottom would fall at the same instant as the top, so there would be no floating-bottom moment.
- Slinky: soft, compliant coils, slow wave, visible delay
- Rigid rod or chain: effectively instantaneous force transmission, no delay
- The floating effect depends on the medium being springy, not on gravity
- 07Apply It to a Different Hanging ChaininteractiveTransfer
Present a transfer scenario: a heavy chain is held by its top link and released. Ask the learner to predict whether the bottom link will hover briefly or fall immediately, using the wave idea.
- Identify the springiness of the medium
- Predict bottom-link behavior at the moment of release
- Connect back to the wave-propagation explanation
- 08Why the Bottom FloatsslideResolution
Directly answer the driving question: the bottom hangs in mid-air because the information that the top has been released travels down the slinky as a wave, and until that wave arrives, the bottom still feels the same upward pull and keeps doing what it was doing.
- Force changes propagate through a spring as a wave, not instantly
- The bottom has no signal to change its motion until the wave reaches it
- Once the wave arrives, the bottom is finally 'in free fall' and joins the top
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