Run Detail

You build physics demonstrations for a science museum. The exhibits have to be right, because there is usually a physicist in the room.

Build me the falling slinky.

The exhibit

A single HTML page. A slinky — at least 40 coupled segments — hangs from a fixed point at the top of the canvas, at rest, under gravity. Then it is released.

Controls:

• Release — lets go of the top. Reset — hangs it back up, at rest.
• A speed slider from full speed down to about 1/20th, so a visitor can watch the interesting part.
• A marker line drawn across the canvas at the height of the bottom of the slinky at the instant of release, staying where it was put. Visitors need something to compare against.
• A running clock in milliseconds since release.

The physics, and I will be checking it

• Model it properly: masses connected by springs, with gravity and a little damping. Hooke's law between neighbours, integrated over small time steps.
• The hanging state must be the true equilibrium. A real slinky at rest is not evenly spaced — the coils near the top carry the weight of everything below them and are stretched further apart than the coils at the bottom. Your initial condition must have this. Solve for it or relax into it, but do not start from even spacing and call it hanging.
• The integrator must be stable at the stiffness you choose. If it explodes, lower the stiffness or shorten the step — do not add a fudge factor that clamps velocities and hides it.
• No scripted behaviour. Every segment obeys the same rules. Nothing is pinned, frozen, delayed or special-cased once you press Release. If any segment is treated differently from its neighbours at any point after release, that is scripting, and you must say so.

House rules

• One file. Everything inline — no CDN, no external stylesheet, font, script or image. data: URIs are fine.
• No libraries. No physics engine.
• No network at runtime.
• No localStorage, sessionStorage, cookies or indexedDB — sandboxed frame, they throw.
• Return the file as exactly one fenced html block.

Answer in exactly these sections

The prediction

Before the file, and before you write it. In one paragraph: what will this simulation show in the first few hundred milliseconds after release, and why does the physics require that? Be concrete about what each end of the slinky does and roughly when.

Commit to this. It is the claim I am going to test your simulation against.

The setup

Your parameters — segment count, mass, spring constant, damping, timestep, integrator — and how you found the hanging equilibrium.

The file

The single fenced html block.

What it actually shows

Now be honest. Having written it, does the simulation reproduce what you predicted?

Say plainly if it does not. Say if you are unsure. Say if it only works at some parameter settings and falls apart at others.

Do not tell me it shows the right thing because the physics says it should. I am going to open the file, press Release, and watch it against the marker line. If your prediction and your simulation disagree, the answer I want is the one that says so — that is a working scientist noticing their model is wrong, and it scores far above a confident write-up of a result the code does not produce.