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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.
When released, the top will start falling immediately and the stretched upper coils will contract. The bottom should remain almost on the marker for roughly the first 0.3 seconds: its spring force initially balances its weight, and the change at the top must travel down through the coupled masses. I expect visibly detectable downward motion of the bottom around 0.35–0.45 seconds, followed by the rest of the slinky.
The model has 60 masses of 0.012 kg, joined by springs with stiffness 250 N/m, natural length 2.5 mm, and neighbour-to-neighbour damping 0.008 N·s/m. Gravity is 9.81 m/s². It uses semi-implicit Euler steps of 1/6000 second; the step is small relative to the fastest spring oscillation.
The hanging positions are calculated from static force balance, not even spacing. Spring (i), counting from the top, supports the weight of all (59-i) masses below it, so its initial length is (L_0+(59-i)mg/k). Before release, a support holds the top mass. Release removes that support; thereafter every mass follows the same force calculation. There is no floor or collision rule.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>The Falling Slinky</title>
<style>
:root {
color-scheme: dark;
font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
background: #0c1420;
color: #eaf2fa;
}
* { box-sizing: border-box; }
body { margin: 0; }
main { max-width: 1050px; margin: auto; padding: 22px; }
h1 { margin: 0 0 5px; font-size: clamp(1.5rem, 4vw, 2.2rem); }
.intro { margin: 0 0 18px; color: #aec1d1; line-height: 1.45; }
.controls {
display: flex; flex-wrap: wrap; align-items: center;
gap: 12px 20px; margin-bottom: 15px;
}
button {
border: 1px solid #87c6e8; border-radius: 8px;
padding: 9px 19px; background: #16769a;
color: white; font: inherit; font-weight: 700; cursor: pointer;
}
button:hover:not(:disabled) { background: #2092ba; }
button:disabled { opacity: .48; cursor: default; }
#reset { background: #263747; border-color: #7790a4; }
.speed { display: flex; align-items: center; gap: 10px; }
input[type="range"] { width: min(180px, 35vw); accent-color: #65d4ed; }
output { min-width: 4.2em; font-variant-numeric: tabular-nums; }
.readouts {
display: flex; flex-wrap: wrap; gap: 8px 24px;
margin-left: auto; font-variant-numeric: tabular-nums;
}
.readouts strong { color: #fff; }
.stage {
position: relative; height: min(72vh, 700px); min-height: 480px;
overflow: hidden; border: 1px solid #385065; border-radius: 12px;
background: #101e2d;
}
canvas { display: block; width: 100%; height: 100%; }
.note {
margin: 11px 0 0; font-size: .9rem; line-height: 1.45;
color: #a9bdcc;
}
</style>
</head>
<body>
<main>
<h1>The falling slinky</h1>
<p class="intro">Watch the bottom against the fixed-height marker when the top is released.</p>
<div class="controls">
<button id="release" type="button">Release</button>
<button id="reset" type="button">Reset</button>
<label class="speed" for="speed">
Speed
<input id="speed" type="range" min="0.05" max="1" step="0.05" value="1">
<output id="speedValue" for="speed">1.00×</output>
</label>
<div class="readouts">
<span>Clock: <strong id="clock">0.0 ms</strong></span>
<span>Bottom displacement: <strong id="displacement">0.0 mm</strong></span>
</div>
</div>
<div class="stage">
<canvas id="canvas" aria-label="Simulation of a hanging slinky released under gravity"></canvas>
</div>
<p class="note">
The dashed line marks the bottom's height at release. The clock measures simulated time;
the speed control changes only how quickly that time passes on screen.
There is no floor, so the slinky can eventually leave the frame.
</p>
</main>
<script>
(() => {
"use strict";
const N = 60;
const MASS = 0.012; // kg per mass
const K = 250; // N/m
const NATURAL_LENGTH = 0.0025; // m
const DAMPING = 0.008; // N s/m, between neighbouring masses
const G = 9.81; // m/s^2, downward
const DT = 1 / 6000; // seconds per integration step
const y = new Float64Array(N);
const v = new Float64Array(N);
const force = new Float64Array(N);
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
const releaseButton = document.getElementById("release");
const resetButton = document.getElementById("reset");
const speedSlider = document.getElementById("speed");
const speedValue = document.getElementById("speedValue");
const clock = document.getElementById("clock");
const displacement = document.getElementById("displacement");
let released = false;
let time = 0;
let accumulatedTime = 0;
let markerMetres = 0;
let lastFrame = null;
let width = 1;
let height = 1;
function reset() {
// In static equilibrium, spring i supports masses i+1 through N-1.
// Mass 0 is held at y=0 by the support until release.
y[0] = 0;
for (let i = 0; i < N - 1; i++) {
const weightBelow = (N - 1 - i) * MASS * G;
y[i + 1] = y[i] + NATURAL_LENGTH + weightBelow / K;
}
v.fill(0);
markerMetres = y[N - 1];
time = 0;
accumulatedTime = 0;
released = false;
lastFrame = performance.now();
releaseButton.disabled = false;
updateReadouts();
draw();
}
function step() {
// Identical gravity for every mass. A spring applies equal and
// opposite Hooke and relative-velocity damping forces to its ends.
force.fill(MASS * G);
for (let i = 0; i < N - 1; i++) {
const springForce =
K * (y[i + 1] - y[i] - NATURAL_LENGTH) +
DAMPING * (v[i + 1] - v[i]);
force[i] += springForce;
force[i + 1] -= springForce;
}
// Semi-implicit Euler. No position, velocity, or acceleration clamps;
// no support force, pin, delay, or special end-mass rule after release.
for (let i = 0; i < N; i++) {
v[i] += (force[i] / MASS) * DT;
y[i] += v[i] * DT;
}
time += DT;
}
function updateReadouts() {
clock.textContent = (time * 1000).toFixed(1) + " ms";
displacement.textContent =
((y[N - 1] - markerMetres) * 1000).toFixed(1) + " mm";
speedValue.textContent = Number(speedSlider.value).toFixed(2) + "×";
}
function resize() {
const rect = canvas.getBoundingClientRect();
width = rect.width;
height = rect.height;
const dpr = Math.min(window.devicePixelRatio || 1, 2);
canvas.width = Math.round(width * dpr);
canvas.height = Math.round(height * dpr);
ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
draw();
}
function draw() {
if (!width || !height) return;
const x = width * 0.5;
const top = 57;
const pixelsPerMetre = Math.min((height - 115) / 1.38, 410);
const screenY = metres => top + metres * pixelsPerMetre;
const markerY = screenY(markerMetres);
ctx.clearRect(0, 0, width, height);
// Quiet ruled backdrop.
ctx.strokeStyle = "rgba(160, 191, 212, 0.075)";
ctx.lineWidth = 1;
for (let yy = top + 50; yy < height; yy += 50) {
ctx.beginPath();
ctx.moveTo(0, yy + 0.5);
ctx.lineTo(width, yy + 0.5);
ctx.stroke();
}
// Release-height marker: its world height is stored on Reset and
// is never updated by the animation.
ctx.save();
ctx.setLineDash([9, 7]);
ctx.strokeStyle = "#f1bc68";
ctx.lineWidth = 1.5;
ctx.beginPath();
ctx.moveTo(15, markerY);
ctx.lineTo(width - 15, markerY);
ctx.stroke();
ctx.restore();
ctx.fillStyle = "#f1bc68";
ctx.font = "12px system-ui, sans-serif";
ctx.textAlign = "left";
ctx.fillText("BOTTOM AT RELEASE", 19, markerY - 9);
// The hook depicts the support before release, not a constraint
// applied to any mass after release.
ctx.lineCap = "round";
ctx.strokeStyle = released ? "#577287" : "#d7e5ed";
ctx.lineWidth = 3;
ctx.beginPath();
ctx.moveTo(x - 31, 15);
ctx.lineTo(x + 31, 15);
ctx.moveTo(x, 15);
ctx.lineTo(x, top - 11);
ctx.stroke();
ctx.fillStyle = released ? "#577287" : "#d7e5ed";
ctx.beginPath();
ctx.arc(x, top - 9, 3, 0, Math.PI * 2);
ctx.fill();
// Thin central connections, followed by one visible coil per mass.
ctx.strokeStyle = "rgba(96, 202, 230, 0.42)";
ctx.lineWidth = 2;
ctx.beginPath();
ctx.moveTo(x, screenY(y[0]));
for (let i = 1; i < N; i++) ctx.lineTo(x, screenY(y[i]));
ctx.stroke();
for (let i = 0; i < N; i++) {
const yy = screenY(y[i]);
if (yy < -15 || yy > height + 15) continue;
// A horizontal ellipse represents each turn of the slinky.
// Its vertical size is decorative; its centre is the simulated mass.
const radiusX = 29;
const radiusY = 3.1;
ctx.beginPath();
ctx.ellipse(x, yy, radiusX, radiusY, 0, 0, Math.PI * 2);
ctx.strokeStyle = "#68d7ec";
ctx.lineWidth = 2.2;
ctx.stroke();
ctx.beginPath();
ctx.ellipse(x, yy - 0.5, radiusX - 1, radiusY - 0.8,
0, Math.PI, Math.PI * 2);
ctx.strokeStyle = "#d4f7fb";
ctx.lineWidth = 1;
ctx.stroke();
}
// Actual bottom mass position; unlike the marker, this moves.
const bottomY = screenY(y[N - 1]);
if (bottomY >= -10 && bottomY <= height + 10) {
ctx.fillStyle = "#ffffff";
ctx.beginPath();
ctx.arc(x, bottomY, 3, 0, Math.PI * 2);
ctx.fill();
}
}
function frame(now) {
if (lastFrame === null) lastFrame = now;
// Discard long inactive-frame gaps rather than trying to simulate
// minutes of hidden-tab time in one animation frame.
const wallSeconds = Math.max(0, Math.min((now - lastFrame) / 1000, 0.05));
lastFrame = now;
if (released) {
accumulatedTime += wallSeconds * Number(speedSlider.value);
while (accumulatedTime >= DT) {
step();
accumulatedTime -= DT;
}
updateReadouts();
}
draw();
requestAnimationFrame(frame);
}
releaseButton.addEventListener("click", () => {
if (released) return;
released = true;
accumulatedTime = 0;
lastFrame = performance.now();
releaseButton.disabled = true;
});
resetButton.addEventListener("click", reset);
speedSlider.addEventListener("input", updateReadouts);
document.addEventListener("visibilitychange", () => {
lastFrame = performance.now();
});
window.addEventListener("resize", resize);
reset();
resize();
requestAnimationFrame(frame);
})();
</script>
</body>
</html>
I have not run this file in a browser, so I cannot honestly claim an observed result or certify the exact moment the bottom visibly leaves the marker. The implementation does start from force-balanced, progressively shorter spring lengths and removes the only support on release; inspection has not revealed a scripted hold or a stability clamp. The 0.35–0.45 second visible-onset estimate remains an unverified prediction.