Edit in JSFiddle

/*
The following is not free software. You may use it for educational purposes, but you may not redistribute or use it commercially.
(C) Burak Kanber 2012
*/

var canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),
    height = 400,
    width = 400,
    x = 200,
    y = 0,
    vy = 0,
    ay = 0,
    m = 0.1,    // Ball mass in kg
    r = 10,     // Ball radius in cm, or pixels.
    dt = 0.02,  // Time step.
    e = -0.5,   // Coefficient of restitution ("bounciness")
    rho = 1.2,  // Density of air. Try 1000 for water.
    C_d = 0.47, // Coeffecient of drag for a ball
    A = Math.PI * r * r / 10000 // Frontal area of the ball; divided by 10000 to compensate for the 1px = 1cm relation
    ;

ctx.fillStyle = 'red';

function loop()
{ 
    var fy = 0;
    
    /* Weight force, which only affects the y-direction (because that's the direction gravity points). */
    fy += m * 9.81;
    
    /* Air resistance force; this would affect both x- and y-directions, but we're only looking at the y-axis in this example. */
    fy += -1 * 0.5 * rho * C_d * A * vy * vy;
    
    /* Verlet integration for the y-direction */
    dy = vy * dt + (0.5 * ay * dt * dt);
    /* The following line is because the math assumes meters but we're assuming 1 cm per pixel, so we need to scale the results */
    y += dy * 100;
    new_ay = fy / m;
    avg_ay = 0.5 * (new_ay + ay);
    vy += avg_ay * dt;
    
    /* Let's do very simple collision detection */
    if (y + r > height && vy > 0)
    {
        /* This is a simplification of impulse-momentum collision response. e should be a negative number, which will change the velocity's direction. */
        vy *= e; 
        /* Move the ball back a little bit so it's not still "stuck" in the wall. */
        y = height - r;                        
    }
    
    draw();
}

function draw()
{
    ctx.clearRect(0, 0, width, height);
    ctx.beginPath();
    ctx.arc(x, y, r, 0, Math.PI * 2, true);
    ctx.fill();
    ctx.closePath();
}
   
/* A real project should use requestAnimationFrame, and you should time the frame rate and pass a variable "dt" to your physics function. This is just a simple brute force method of getting it done. */
setInterval(loop, dt * 1000);

<canvas id="canvas" height="400" width="400"></canvas>

canvas {
    display:block;
    margin:10px auto;
    border:1px solid #999;
}