var l, normal;

var v = 0;

var g = 5;

var k = 0.01;

​

function setup() {

  createCanvas(400, 400);

  normal = height/2;

  l = normal;

}

​

function draw() {

  background(240);

  line(width/2, 0, width/2, l);

  circle(width/2, l, r);

  l += v;

  v += g;

  v -= k * (l-normal);

}