_bodiesApplyGravity = function(bodies, gravity) {
        var gravityScale = typeof gravity.scale !== 'undefined' ? gravity.scale : 0.001;

        if ((gravity.x === 0 && gravity.y === 0) || gravityScale === 0) {
            return;
        }
        
        for (var i = 0; i < bodies.length; i++) {
            var body = bodies[i];

            if (body.isStatic || body.isSleeping)
                continue;

            // apply gravity
            body.force.y += body.mass * gravity.y * gravityScale;
            body.force.x += body.mass * gravity.x * gravityScale;
        }
    }
Applys a mass dependant force to all given bodies.
@method bodiesApplyGravity
@private
@param {body[]} bodies
@param {vector} gravity