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The Falling Slinky

Why the Bottom Can Hang Still After the Top Is Released

Big Question: How can the bottom experience gravity yet remain nearly at rest until information about the release reaches it through the spring?

Quick Answer

Before release, each part of the hanging slinky is already under tension. At the bottom, the local spring tension balances the weight of the material below it, which approaches zero at the very bottom. Higher up, greater tension supports more of the slinky.

When the top is released, the tension there suddenly drops. That change travels downward through the spring as a disturbance or collapse front. Below the front, the slinky initially retains almost the same tension distribution it had while hanging, so gravity remains locally balanced by spring forces and the bottom stays nearly stationary. Above the front, coils collapse and accelerate downward. Only when the front reaches the bottom does its force balance change strongly and the bottom begins to fall.

High-speed experiments and models reported by Cross and Wheatland in 2012 reproduce this behaviour, including the finite collapse time of the coils behind the front.

University of Sydney — The Falling Slinky →

What You Will Learn

• Why a hanging slinky has a tension gradient.
• Why gravity alone does not determine local acceleration.
• How spring forces balance gravity before release.
• Why releasing the top changes a boundary condition.
• Why the tension change propagates at finite speed.
• What a collapse front is.
• Why the bottom can remain nearly stationary.
• Why the centre of mass still accelerates downward normally.
• Why this does not violate causality or gravity.
• How coil collisions modify a simple elastic-wave model.
• What high-speed video measures.
• How this reasoning transfers to ropes, rods and elastic structures.

Part 1 — The Naive Model: Everything Falls at Once

For a point mass released near Earth’s surface, acceleration is approximately g downward.

A slinky is not a point mass. It is an extended object with internal forces.

Each small section is pulled by gravity and by neighbouring sections of spring. Its acceleration depends on the net force, not gravity alone.

Part 2 — Before Release, Every Section Is in Force Balance

While the slinky hangs motionless, acceleration is zero everywhere.

Therefore the net force on each small section must be zero. Gravity pulls downward; differences in spring tension provide the balancing upward force.

The top must support almost the entire slinky, so tension is largest there. Tension decreases downward because each lower point supports less material.

Part 3 — Releasing the Top Changes Only One Place First

At the instant your hand lets go, the top boundary changes from “supported” to “free.”

The top tension drops rapidly. But a distant section cannot know about that change before an elastic disturbance reaches it.

forces inside an extended material are communicated by waves at finite speed.

Part 4 — The Release Disturbance Travels Downward

The sudden loss of top tension launches a disturbance down the stretched slinky.

Behind this front, the coils contract strongly toward their natural close-packed spacing. Ahead of it, the slinky remains close to its original stretched state.

The front therefore separates two very different mechanical regions.

Part 5 — Why the Bottom Initially Stays Put

Before the wave arrives, the bottom region still has nearly the same local extension and tension it had while hanging.

The upward spring-force gradient therefore continues to balance gravity there to a good approximation.

No large downward net force appears at the bottom until the collapse front arrives.

Part 6 — The Top Falls Faster Than You Might Expect

Once released, the upper coils are pulled downward not only by gravity but also by the contracting stretched spring below them.

They therefore collapse rapidly, gathering into a denser packet that moves downward toward the stationary lower end.

The visual impression of the top “catching” the bottom is a consequence of this redistribution of internal force and mass.

Part 7 — The Centre of Mass Still Obeys External Force

The entire slinky’s centre of mass accelerates downward because gravity is the dominant external force after release.

Internal spring forces can redistribute acceleration among different parts, but they cancel in the total centre-of-mass force balance.

So the stationary bottom is not “escaping gravity.” The upper mass simply accelerates in a way that keeps the overall centre-of-mass motion consistent with Newton’s laws.

Part 8 — Why a Simple Uniform Spring Model Is Not Perfect

A real slinky has coils that can collide and stack once they contract.

This introduces a strongly nonlinear collapse front rather than a small-amplitude wave moving through an ideal Hookean spring forever.

Cross and Wheatland included finite collapse time behind the front to improve agreement with high-speed data.

Part 9 — Hooke’s Law Is Not Being Broken

Hooke’s law is a local approximation relating force and extension over an elastic regime.

The surprise comes from applying that elasticity to an extended object with a pre-existing tension gradient and a propagating change in boundary conditions.

The phenomenon therefore extends ordinary spring physics rather than contradicting it.

Part 10 — Information Does Not Travel Instantly Through Matter

If removing the top support could change the bottom tension instantaneously, the bottom would begin responding immediately.

Real materials communicate mechanical changes through elastic disturbances whose speeds depend on stiffness, density and geometry.

The falling slinky is therefore a visible demonstration of finite signal speed in classical mechanics.

Part 11 — The Failed Model → The Better Model

Naive model	Why it fails	Better model
Gravity acts everywhere, so every part accelerates at g immediately.	Internal spring forces also act and may initially balance gravity locally.	Use the net-force field along the extended object.
Releasing the top instantly removes all tension.	A boundary-condition change propagates at finite speed.	Track a release/collapse wave.
The bottom staying still violates free fall.	The centre of mass still accelerates under gravity.	Separate local motion from whole-system motion.
An ideal linear spring explains every detail.	Real coils collide and form a nonlinear collapse front.	Include finite coil-collapse dynamics.

Part 12 — How Do We Know?

• High-speed video tracks top, bottom and collapse-front positions.
• Frame-by-frame measurements show the bottom remains nearly stationary before front arrival.
• Changing slinky mass and stiffness changes front propagation.
• Models using the initial hanging tension distribution predict the delayed response.
• Centre-of-mass motion can be reconstructed and checked against gravity.
• Comparing ideal wave models with coil-collision models tests which details require nonlinearity.

Observation vs Inference

• Observation: the bottom remains nearly fixed after release until the collapse front reaches it.
• Measurement: a front travels downward through the slinky.
• Static fact: the hanging slinky begins with a non-uniform tension distribution.
• Inference: the bottom retains near-balanced forces until the release information arrives.
• Boundary: exact front shape and speed depend on the slinky’s constitutive behaviour and coil collisions.

Checkpoint Questions

1. Why is tension greatest near the top of a hanging slinky?
2. Why is the net force zero before release?
3. What changes when the top is released?
4. Why does the bottom not know immediately?
5. What is the collapse front?
6. Why can the bottom remain stationary while gravity acts?
7. What happens to the centre of mass?
8. Why can the top accelerate rapidly?
9. Why are coil collisions important?
10. What observation most directly falsifies the “all parts free-fall immediately” model?

Answer Key

Open after attempting the questions

1. It supports the weight of almost all the slinky below it.
2. Spring-force differences balance gravity in static equilibrium.
3. The supported boundary becomes free and top tension collapses.
4. The tension change propagates through the elastic material at finite speed.
5. The moving boundary between collapsed upper coils and still-stretched lower coils.
6. Its original spring-force balance remains almost unchanged until the front arrives.
7. It accelerates downward according to the net external force on the whole slinky.
8. Gravity and contraction of the stretched spring both contribute to upper-coil motion.
9. They turn small-strain wave behaviour into a nonlinear compacting front.
10. High-speed video showing negligible bottom displacement before front arrival.

Primary Science Bridge

• gravity is not the only force on an object;
• springs pull when stretched;
• balanced forces can produce no acceleration;
• changes travel through materials;
• different parts of one object can move differently.

Secondary and JC Bridge

Core idea	Higher-resolution route
Forces	Distributed tension gradient
Springs	Continuum elastic models
Waves	Finite-speed stress information
Momentum	Centre-of-mass motion
Nonlinearity	Coil collision and collapse shocks
Boundary conditions	Release-driven transient dynamics

Unfamiliar Transfer Challenge

A long elastic cable hangs vertically from a crane. The support is suddenly released. A sensor near the bottom initially reports almost no acceleration.

Do not conclude that gravity is absent. Measure the pre-release tension profile, elastic-wave speed and arrival time of the release disturbance. Then check whether the local acceleration changes when the stress wave reaches the sensor.

Deep Science Window — Static Tension Gradient

For a vertically hanging continuum, tension at a given height equals the weight of material below that point in static equilibrium. Differentiating that tension with position produces exactly the upward force density needed to balance gravity. The lower region therefore begins with a mechanically encoded memory of the original support condition.

Deep Science Window — Characteristics and Signal Speed

Elastic continuum equations transmit disturbances along characteristic paths at finite wave speeds. Boundary information cannot leap arbitrarily far ahead of those characteristics. The slinky gives an unusually visible example because the front is slow enough to watch directly.

Evidence Boundaries

• Bottom stationary ≠ gravity switched off.
• Gravity everywhere ≠ local acceleration g everywhere.
• Release at the top ≠ instant tension loss at the bottom.
• Centre-of-mass free fall ≠ every part follows the same trajectory.
• Ideal Hookean spring ≠ exact real slinky collapse.
• Finite mechanical signal speed ≠ relativistic effect; it is ordinary elasticity and material inertia.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: tension, static equilibrium, elastic wave, collapse front, net force, centre of mass.

CONNECT: pre-release tension gradient to local force balance, release to a travelling stress change, and front arrival to the onset of bottom motion.

EXPLAIN: why the bottom can stay nearly still even though gravity acts on it.

APPLY: predict delayed local response in another long elastic structure.

CHECK: compare release-wave arrival time with the time local acceleration changes.

Research Sources and Further Reading

• University of Sydney — The Falling Slinky
• W. G. Unruh — The Falling Slinky