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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.
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:
data: URIs are fine.localStorage, sessionStorage, cookies or indexedDB — sandboxed frame, they throw.html block.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.
Your parameters — segment count, mass, spring constant, damping, timestep, integrator — and how you found the hanging equilibrium.
The single fenced html block.
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.
At the instant of release the top coil is the most violently unbalanced object in the system: the anchor spring above it was carrying the entire weight of the slinky (≈ N·m·g) and that force vanishes, while the spring below it still pulls down with ≈ (N−1)·m·g. So mass 1 starts with a downward acceleration of about N·g ≈ 60 g, not g, and it slams into coil 2 within a few milliseconds; that pair then hits coil 3, and so on. What propagates downward is a collapse front — above it the coils are bunched to their closed length and are all falling together; below it the slinky is still stretched in its original hanging profile and, crucially, has not moved at all, because no signal can reach it faster than the front. The bottom coil should therefore sit dead on the marker line — motionless to within a fraction of a pixel — for roughly the first 250–300 ms, while the top plummets through most of the slinky's ~1.1 m length. Meanwhile the centre of mass falls with exactly g from t = 0 (all internal forces cancel), which is what forces the top to move so much faster than g while the bottom is stationary. When the front reaches the last coil (my estimate: ≈ 270 ms, from the CoM having to fall the ~0.35 m from its hanging position to the bottom of the slinky), the bottom is jerked into motion abruptly — it does not accelerate smoothly from rest, it acquires most of its speed in a few milliseconds — and from then on the whole bunched slinky falls as one lump at g, with some residual ringing of the compressed coils. Expect the top to cross the marker line at around 250 ms, i.e. very nearly when the bottom starts to move.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>The Falling Slinky</title>
<style>
html,body{margin:0;padding:0;background:#11141a;color:#dfe6f2;
font-family:ui-monospace,Menlo,Consolas,monospace;}
#wrap{display:flex;gap:18px;padding:16px;align-items:flex-start;}
canvas{background:#0b0d12;border:1px solid #2a3140;border-radius:6px;}
#panel{width:330px;font-size:13px;line-height:1.5;}
h1{font-size:17px;margin:0 0 4px 0;letter-spacing:.5px;}
.sub{color:#8b97ab;font-size:12px;margin-bottom:14px;}
button{font:inherit;background:#1d2532;color:#dfe6f2;border:1px solid #3a4considered;}
button{border:1px solid #3a4557;border-radius:5px;padding:7px 14px;cursor:pointer;}
button:hover{background:#28324a;}
button:active{background:#334160;}
.row{margin:10px 0;}
input[type=range]{width:100%;}
table{border-collapse:collapse;width:100%;margin-top:10px;font-size:12.5px;}
td{padding:3px 4px;border-bottom:1px solid #1e2431;}
td.k{color:#8b97ab;}
td.v{text-align:right;font-variant-numeric:tabular-nums;}
.big{font-size:26px;font-variant-numeric:tabular-nums;color:#ffd479;}
.note{color:#75melt;}
.note{color:#7581)}
.note{color:#758197;font-size:11.5px;margin-top:14px;line-height:1.45;}
.legend span{display:inline-block;margin-right:12px;}
.sw{display:inline-block;width:14px;height:3px;vertical-align:middle;margin-right:4px;}
</style>
</head>
<body>
<div id="wrap">
<canvas id="cv" width="860" height="720"></canvas>
<div id="panel">
<h1>THE FALLING SLINKY</h1>
<div class="sub">60 masses · Hooke springs · symplectic Euler, dt = 100 µs</div>
<div class="row">
<button id="btnRel">Release</button>
<button id="btnRes">Reset</button>
</div>
<div class="row">
<label for="spd">Speed: <span id="spdLbl">1/1</span> of real time</label>
<input id="spd" type="range" min="0" max="100" value="100">
</div>
<div class="row">
<div class="k" style="color:#8b97ab">clock since release</div>
<div class="big"><span id="clk">0.0</span> ms</div>
</div>
<table>
<tr><td class="k">top coil, moved</td><td class="v"><span id="dTop">0</span> mm</td></tr>
<tr><td class="k">bottom coil, moved</td><td class="v"><span id="dBot">0</span> mm</td></tr>
<tr><td class="k">bottom speed</td><td class="v"><span id="vBot">0</span> mm/s</td></tr>
<tr><td class="k">centre of mass, moved</td><td class="v"><span id="dCom">0</span> mm</td></tr>
<tr><td class="k">CoM minus ½gt²</td><td class="v"><span id="eCom">0</span> mm</td></tr>
<tr><td class="k">collapsed coils</td><td class="v"><span id="nCol">0</span> / 60</td></tr>
</table>
<div class="note legend" style="margin-top:12px">
<span><i class="sw" style="background:#ff5d6c"></i>marker: bottom at release</span><br>
<span><i class="sw" style="background:#4fa3ff"></i>free-fall CoM, ½gt²</span>
</div>
<div class="note">
Hanging state is the exact analytic equilibrium: tension in spring <i>i</i> equals the weight
hanging below it, so the top gap is 32.7 mm and the bottom gap 0.55 mm.
Release removes the anchor spring only. After that every mass obeys the identical force law
(gravity + spring above + spring below + dashpot); mass 1 simply has no spring above it,
exactly as mass 60 has none below. No clamping, no scripting.
</div>
</div>
</div>
<script>
"use strict";
// ---------------------------------------------------------------- parameters
const N = 60; // number of point masses
const Mtot = 0.2; // kg, total slinky mass
const m = Mtot / N; // kg per segment
const g = 9.81; // m/s^2
const k = 60.0; // N/m per segment spring (=> ~1 N/m whole slinky)
const kc = 6.0 * k; // N/m when coils are in contact (compressed)
const L0 = 0.002; // m, closed length of one coil
const cDamp = 0.02; // N.s/m dashpot in each spring (~3% critical)
const dt = 1.0e-4; // s, fixed physics step
// ---------------------------------------------------------------- state
const y = new Float64Array(N + 1); // y[0] = anchor, downward positive, metres
const v = new Float64Array(N + 1);
const f = new Float64Array(N + 1);
let held = true; // anchor spring present?
let t = 0; // seconds since release
let markerY = 0; // bottom position at release (m)
let topY0 = 0, botY0 = 0, comY0 = 0;
function equilibrium() {
// Exact static solution: tension in spring i supports masses i..N.
y[0] = 0; v[0] = 0;
for (let i = 1; i <= N; i++) {
const T = m * g * (N - i + 1);
y[i] = y[i - 1] + L0 + T / k;
v[i] = 0;
}
}
function com() {
let s = 0;
for (let i = 1; i <= N; i++) s += y[i];
return s / N;
}
function reset() {
equilibrium();
held = true;
t = 0;
topY0 = y[1]; botY0 = y[N]; comY0 = com();
markerY = y[N];
}
function release() {
if (!held) return;
held = false;
t = 0;
topY0 = y[1]; botY0 = y[N]; comY0 = com();
markerY = y[N]; // marker = bottom of slinky at the instant of release
}
// ---------------------------------------------------------------- physics
function step() {
for (let i = 1; i <= N; i++) f[i] = m * g; // gravity, downward positive
const first = held ? 1 : 2; // spring 1 is the anchor spring
for (let i = first; i <= N; i++) {
const ya = (i === 1) ? 0 : y[i - 1];
const va = (i === 1) ? 0 : v[i - 1];
const ext = (y[i] - ya) - L0; // + = stretched
const kk = (ext >= 0) ? k : kc; // coils in contact are stiffer
const Fs = kk * ext + cDamp * (v[i] - va); // + = pulls i up, pulls i-1 down
f[i] -= Fs;
if (i > 1) f[i - 1] += Fs;
}
for (let i = 1; i <= N; i++) { // semi-implicit Euler
v[i] += (f[i] / m) * dt;
y[i] += v[i] * dt;
}
if (!held) t += dt;
}
// ---------------------------------------------------------------- rendering
const cv = document.getElementById("cv");
const ctx = cv.getContext("2d");
const W = cv.width, H = cv.height;
const CX = 300; // slinky axis, px
const TOPPX = 42; // anchor position, px
const SCALE = 380; // px per metre
const py = yy => TOPPX + yy * SCALE;
function draw() {
ctx.clearRect(0, 0, W, H);
// depth grid
ctx.strokeStyle = "#161b25"; ctx.lineWidth = 1;
for (let d = 0; d <= 1.8; d += 0.1) {
const Y = py(d);
if (Y > H) break;
ctx.beginPath(); ctx.moveTo(0, Y); ctx.lineTo(W, Y); ctx.stroke();
}
ctx.fillStyle = "#3d4759"; ctx.font = "11px ui-monospace,monospace";
for (let d = 0; d <= 1.8; d += 0.5) {
const Y = py(d); if (Y > H) break;
ctx.fillText(d.toFixed(1) + " m", 6, Y - 4);
}
// ceiling / anchor
ctx.strokeStyle = "#4d586c"; ctx.lineWidth = 3;
ctx.beginPath(); ctx.moveTo(CX - 120, TOPPX); ctx.lineTo(CX + 120, TOPPX); ctx.stroke();
ctx.lineWidth = 1; ctx.strokeStyle = "#2d3648";
for (let x = CX - 116; x < CX + 118; x += 12) {
ctx.beginPath(); ctx.moveTo(x, TOPPX); ctx.lineTo(x - 8, TOPPX - 9); ctx.stroke();
}
if (held) {
ctx.strokeStyle = "#6f7c93"; ctx.lineWidth = 2;
ctx.beginPath(); ctx.moveTo(CX, TOPPX); ctx.lineTo(CX, py(y[1])); ctx.stroke();
}
// marker line: bottom of slinky at instant of release
const mY = py(markerY);
ctx.strokeStyle = "#ff5d6c"; ctx.lineWidth = 1.5;
ctx.setLineDash([9, 6]);
ctx.beginPath(); ctx.moveTo(0, mY); ctx.lineTo(W, mY); ctx.stroke();
ctx.setLineDash([]);
ctx.fillStyle = "#ff8b96"; ctx.font = "12px ui-monospace,monospace";
ctx.fillText("bottom of slinky at release", W - 210, mY - 7);
// free-fall reference for the centre of mass
if (!held) {
const cY = py(comY0 + 0.5 * g * t * t);
if (cY < H) {
ctx.strokeStyle = "#4fa3ff"; ctx.setLineDash([3, 7]); ctx.lineWidth = 1.5;
ctx.beginPath(); ctx.moveTo(0, cY); ctx.lineTo(W, cY); ctx.stroke();
ctx.setLineDash([]);
ctx.fillStyle = "#79bcff";
ctx.fillText("free-fall CoM", W - 210, cY - 7);
}
}
// the slinky: one flat ellipse per coil
ctx.lineWidth = 2;
for (let i = 1; i <= N; i++) {
const Y = py(y[i]);
if (Y < -20 || Y > H + 20) continue;
const frac = (i - 1) / (N - 1);
ctx.strokeStyle = "rgb(" + Math.round(120 + 110 * frac) + "," +
Math.round(200 - 40 * frac) + "," +
Math.round(255 - 90 * frac) + ")";
ctx.beginPath();
ctx.ellipse(CX, Y, 62, 6.5, 0, 0, Math.PI * 2);
ctx.stroke();
}
// highlight the two ends
const drawEnd = (i, col, lbl) => {
const Y = py(y[i]); if (Y < -20 || Y > H + 20) return;
ctx.strokeStyle = col; ctx.lineWidth = 3;
ctx.beginPath(); ctx.ellipse(CX, Y, 62, 6.5, 0, 0, Math.PI * 2); ctx.stroke();
ctx.fillStyle = col; ctx.font = "12px ui-monospace,monospace";
ctx.fillText(lbl, CX + 74, Y + 4);
};
drawEnd(1, "#ffd479", "top");
drawEnd(N, "#8affc1", "bottom");
}
// ---------------------------------------------------------------- ui / loop
const spd = document.getElementById("spd");
const spdLbl = document.getElementById("spdLbl");
function speedFactor() {
// slider 0..100 -> 1/20 .. 1 (log)
const u = spd.value / 100;
return Math.pow(20, u) / 20;
}
function updSpdLbl() {
const s = speedFactor();
spdLbl.textContent = (s > 0.995) ? "1/1" : "1/" + (1 / s).toFixed(1);
}
spd.addEventListener("input", updSpdLbl);
document.getElementById("btnRel").addEventListener("click", release);
document.getElementById("btnRes").addEventListener("click", reset);
window.addEventListener("keydown", e => {
if (e.code === "Space") { e.preventDefault(); release(); }
if (e.code === "KeyR") { reset(); }
});
const el = id => document.getElementById(id);
function readouts() {
el("clk").textContent = (t * 1000).toFixed(1);
el("dTop").textContent = ((y[1] - topY0) * 1000).toFixed(1);
el("dBot").textContent = ((y[N] - botY0) * 1000).toFixed(3);
el("vBot").textContent = (v[N] * 1000).toFixed(1);
const c = com();
el("dCom").textContent = ((c - comY0) * 1000).toFixed(1);
el("eCom").textContent = ((c - comY0 - 0.5 * g * t * t) * 1000).toFixed(4);
let nc = 0;
for (let i = 2; i <= N; i++) if (y[i] - y[i - 1] < 1.5 * L0) nc++;
el("nCol").textContent = nc;
}
let acc = 0, last = performance.now();
function frame(now) {
let real = (now - last) / 1000; last = now;
if (real > 0.05) real = 0.05;
acc += real * speedFactor();
let steps = Math.floor(acc / dt);
if (steps > 1200) steps = 1200; // keep the tab responsive
acc -= steps * dt;
for (let s = 0; s < steps; s++) step();
draw();
readouts();
requestAnimationFrame(frame);
}
reset();
updSpdLbl();
requestAnimationFrame(frame);
</script>
</body>
</html>
Honest position first: I have not been able to execute this file, so what follows is what I expect from the numbers, flagged where I am unsure. Please check me against the readouts, which I put there specifically so you can catch me — bottom coil, moved and CoM minus ½gt² are the two lines that will convict the model if it is wrong.
What I am confident about: the hanging state is the exact analytic equilibrium (net force is identically zero mass-by-mass, so it should not creep at all before you press Release), and the top-coil acceleration at t = 0⁺ is N·g = 589 m/s², which is arithmetic, not hope. The bottom coil cannot move before the front arrives — that is causality in a nearest-neighbour system — so bottom coil, moved should read 0.000 mm for the first ~200 ms and the last coil should sit on the red marker. CoM minus ½gt² should stay at the 10⁻⁴ mm level until coils leave the frame, since there is no drag and internal forces cancel; if that number drifts, the integrator is leaking energy and you should tell me.
What I am less sure of: the exact collapse time. My 270 ms comes from a centre-of-mass argument, and the true number depends on how much the bunched region overshoots and rings against the 6k contact springs. I would not be shocked to see anything in the 230–320 ms window, and if it lands well outside that, my estimate was wrong, not the code. I am also unsure how clean the arrival at the bottom looks: with linear compression springs rather than a true rigid contact, the collapsed clump will ring, and there is a possibility that a small precursor of that ringing reaches the bottom slightly ahead of the main front and nudges it by a few hundredths of a millimetre before the visible jerk. That would show as bottom coil, moved reading 0.01–0.05 mm rather than exactly zero in the 150–250 ms range. That is real physics in the model, not a bug, but it is not quite the "absolutely nothing happens" that the demo narrative claims, and you should see it in the third decimal place.
Known limitations: the slinky falls out of the bottom of the frame roughly 350 ms after the bottom starts moving — the camera is deliberately fixed because the marker line has to stay put, so the post-collapse free fall is only briefly visible. At 1/20 speed the physics is unchanged (same dt, fewer steps per frame), so slow motion is faithful rather than a re-render. If you raise k or lower the sub-step count the scheme will eventually go unstable, but at the shipped values dt is 26× inside the linear stability limit and I would expect no growth over the ~1 s of interest; there is no velocity clamp to hide it if I am wrong, only the 1200-step-per-frame cap, which slows the simulation down rather than altering it.