Adding gravity! Here we’ve adjusting the y velocity in every frame with the force of gravity. It’s fun to play with options and see how they are affecting the animation and the physics, so I’ve also got a checkbox that will toggle gravity to either a vlaue of 2 or 0. interactive physics animations via javascript & canvas | 12.

[cc lang=”javascript”]

$(function () {

var canvas, context, width, height, x, y, radius = 25, clickX, clickY, drag = false;

var total_dots = 25;

var fps = 24;

var bounce = -1;

var gravity = 2;

canvas = $(“#canvas”)[0];

context = canvas.getContext(“2d”);

var dots = new Array();

var drag_i = -1;

var this_dot = {};

for (var i=0; i < total_dots; i++){
var this_dot = {
x: Math.random()*canvas.width,
y: Math.random()*canvas.height,
vx: Math.random()*30-10,
vy: Math.random()*30-10,
width:canvas.width,
height: canvas.height,
radius:Math.random()*20+10
};
dots.push(this_dot);
}
draw();
$("#canvas").mousedown(function (event) {
var dx, dy, dist;
for (var i=0; i < dots.length; i++){
dx = event.pageX - this.offsetLeft - dots[i].x;
dy = event.pageY - this.offsetTop - dots[i].y;
dist = Math.sqrt(dx * dx + dy * dy);
if(dist < radius) {
drag = true;
drag_i = i
clickX = dx;
clickY = dy;
continue;
}
}
});
$("#canvas").mouseup(function (event) {
drag = false;
drag_i = -1;
});
$("#canvas").mousemove(function (event) {
if(drag) {
dots[drag_i].x = event.pageX - this.offsetLeft - clickX;
dots[drag_i].y = event.pageY - this.offsetTop - clickY;
draw();
}
});
function update(){
for (var i=0; i < dots.length; i++){
if (drag_i != i){
var this_dot = dots[i];
this_dot.vy = this_dot.vy + gravity;
this_dot.x += this_dot.vx;
this_dot.y += this_dot.vy;
if (this_dot.x > canvas.width – this_dot.radius){

this_dot.x = canvas.width – this_dot.radius;

this_dot.vx = this_dot.vx * bounce;

}

else if(this_dot.x < 0 + this_dot.radius){
this_dot.x = this_dot.radius;
this_dot.vx = this_dot.vx * bounce;
}
if (this_dot.y > canvas.height – this_dot.radius){

this_dot.y = canvas.height – this_dot.radius;

this_dot.vy = this_dot.vy * bounce;

}

else if(this_dot.y < 0 + this_dot.radius){
this_dot.y = this_dot.radius;
this_dot.vy = this_dot.vy * bounce;
}
}
}
}
function draw() {
context.clearRect(0, 0, canvas.width, canvas.height);
for (var i=0; i < dots.length; i++){
context.beginPath();
context.arc(dots[i].x, dots[i].y, dots[i].radius, 0, Math.PI * 2, false);
context.fill();
context.closePath();
}
}
setInterval(function() {
update();
draw();
}, 1000/fps);
$("#gravity").click(function(){
if($("#gravity").is(':checked')){
gravity = 2;
}
else{
gravity = 0;
}
});
});
[/cc]
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