Spring!

gistfile1.js

//Config variables
var rest_length     = 256.0
var spring_constant = 1.0
var dt              = 0.01

//Hooke's law
function force(displacement) {
  return spring_constant * (rest_length - displacement)
}

//State of system
var x = 0.0
var v = 0.0

//Integrate forward by dt
function step() {
  var f = force(x)
  x += (v + f * 0.5 * dt) * dt
  v += (f + force(x)) * 0.5 * dt
}

//Set up canvas
var canvas = document.createElement("canvas")
canvas.width = 512
canvas.height = 512
document.body.appendChild(canvas)
var context = canvas.getContext("2d")

//Animate!
setInterval(function() {
  context.clearRect(0, 0, 512, 512)
  context.fillStyle = "rgba(0,0,0,255)"
  context.beginPath()
  context.arc(x, 256, 10, 0, Math.PI*2, true);
  context.closePath();
  context.fill()
  step()
}, 10)