How to apply gravity to bouncing balls in javascript -

Question

I want to move from having a ball bounce back and forth around a canvas to having some gravity and eventually dropping. I know i need to change the velocity when it reaches

Answer 1

You were very close, think of the gravity as a constant downwards velocity increment, so in each step you need to add that to your vy calculation.

"I know i need to change the velocity when it reaches the bottom"`

That is not true because gravity affects objects ALL the time. When you touch the bottom, things like material dampening and surface friction can happen.

var canvas,
  ctx,
  cx = 100,
  cy = 100,
  vx = 2,
  vy = 5,
  radius = 5,
  gravity = 0.2,
  damping = 0.9,
  traction = 0.8,
  paused = false;
  ;

function init() {

  canvas = document.getElementById("gameCanvas");
  ctx = canvas.getContext("2d");
  
  canvas.width = 300;
  canvas.height = 150;

  circle();
}

function circle() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  if (!paused)
    requestAnimationFrame(circle);

  if (cx + radius >= canvas.width) {
    vx = -vx * damping;
    cx = canvas.width - radius;
  } else if (cx - radius <= 0) {
    vx = -vx * damping;
    cx = radius;
  }
  if (cy + radius >= canvas.height) {
    vy = -vy * damping;
    cy = canvas.height - radius;
    // traction here
    vx *= traction;
  } else if (cy - radius <= 0) {
    vy = -vy * damping;
    cy = radius;
  }

  vy += gravity; // <--- this is it

  cx += vx;
  cy += vy;

  ctx.beginPath();
  ctx.arc(cx, cy, radius, 0, 2 * Math.PI, false);
  ctx.fillStyle = 'green';
  ctx.fill();
}

init();

// fancy/irrelevant mouse grab'n'throw stuff below
canvas.addEventListener('mousedown', handleMouseDown);
canvas.addEventListener('mouseup', handleMouseUp);

function handleMouseDown(e) {
  cx = e.pageX - canvas.offsetLeft;
  cy = e.pageY - canvas.offsetTop;
  vx = vy = 0;
  paused = true;
}

function handleMouseUp(e) {
  vx = e.pageX - canvas.offsetLeft - cx;
  vy = e.pageY - canvas.offsetTop - cy;
  paused = false;
  circle();
}

canvas {border: 1px solid black; cursor: crosshair;}
p {margin: 0;}

<canvas id="gameCanvas"></canvas>
<p>Throw the ball by holding and releasing the left mouse button on the canvas (reverse slingshot)</p>