// rectangle x, y position
var x = 400; // starting x position
var y = 250; // starting y position
var FPS = 60; // frames per second of the screen
// physics variables:
var velX = 0; // initial velocity at 0 (not moving)
var velY = 0; // not moving
var drag = 0.92; // drag force reduces velocity by 8% per frame
var force = 0.35; // overall force applied to move the rectangle
var angle = 0; // angle in which to move

// called every frame (at 60 frames per second):
function update(){
    // calculate distance between mouse and rectangle
    var dx = mouseX - x;
    var dy = mouseY - y;
    // calculate angle between mouse and rectangle
    var angle = Math.atan(dy/dx);
    if(dx < 0)
        angle += Math.PI;
    else if(dy < 0)
        angle += 2*Math.PI;

    // calculate the force (on or off, depending on user input)
    var curForce;
    if(keys[32]) // SPACE bar
        curForce = force; // if pressed, use 0.35 as force
    else
        curForce = 0; // otherwise, force is 0

    // increment velocty by the force, and scaled by drag for x and y
    velX += curForce * Math.cos(angle);
    velX *= drag;
    velY += curForce * Math.sin(angle);
    velY *= drag;

    // update x and y by their velocities
    x += velX;
    y += velY;