[Герыч]

Да, добавь переменные xx,yy в класс Particle и все расчёты веди в них, а потом сливай в x и y - все проблемы пропадут!

Добавлено через 19 минут
Вот, это работает:

package gerich.verlet
{
	import flash.display.Shape;
	import flash.display.Sprite;
	import flash.events.Event;
	import flash.events.MouseEvent;
 
	public class Main extends Sprite
	{
		private var pt:Array = new Array();
		private var particles:Sprite = new Sprite();
		private var canvas:Shape = new Shape();
		private var grav:Number = 5;
		private var timeSqr:Number;
 
		// ========================================================== //
		public function Main():void
		{
			stage.frameRate = 40;
 
			timeSqr=1/stage.frameRate;timeSqr*=timeSqr;
 
			addChild(canvas);
 
			addChild(particles);
 
			pt.push(new Particle(270,150));
			pt.push(new Particle(270,175));
			pt.push(new Particle(270,200));
			pt.push(new Particle(270,225));
			pt.push(new Particle(270,250));
			pt.push(new Particle(270,275));
			pt.push(new Particle(255,285));
			pt.push(new Particle(285,285));
 
			for (var i in pt)
			{
				particles.addChild(pt[i]);
			}
 
			addEventListener(Event.ENTER_FRAME, checkJoints);
			addEventListener(Event.ENTER_FRAME, ptMouse);
		}
		// ========================================================== //
		private function checkJoints(event:Event):void
		{
			var dx:Number;
			var dy:Number;
 
			canvas.graphics.clear();
 
			// Gravity
 
			for (var i in pt)
			{
				dx = (pt[i].xx - pt[i].old_x);
				dy = (pt[i].yy - pt[i].old_y);
				pt[i].old_x = pt[i].xx;
				pt[i].old_y = pt[i].yy;
				pt[i].xx += dx;
				pt[i].yy += dy+grav*timeSqr;
			}
 
			// Joints
 
			for (var z:int=0; z<5; z++)
			{
				mouseJoint(pt[0], pt[1], 25);
				ptJoint(pt[1], pt[2], 25);
				ptJoint(pt[2], pt[3], 25);
				ptJoint(pt[3], pt[4], 25);
				ptJoint(pt[4], pt[5], 25);
				ptJoint(pt[5], pt[6], 25);
				ptJoint(pt[5], pt[7], 25);
				ptJoint(pt[6], pt[7], 25);
			}
 
			for (var i in pt)
			{
				pt[i].x = pt[i].xx;
				pt[i].y = pt[i].yy;
			}
			// Lines
 
			drawLine(pt[1], pt[2]);
			drawLine(pt[2], pt[3]);
			drawLine(pt[3], pt[4]);
			drawLine(pt[4], pt[5]);
			drawLine(pt[5], pt[6]);
			drawLine(pt[5], pt[7]);
			drawLine(pt[6], pt[7]);
		}
		// ========================================================== //
		private function ptMouse(event:Event):void
		{
			pt[0].xx = mouseX;
			pt[0].yy = mouseY;
			pt[0].x = mouseX;
			pt[0].y = mouseY;
			drawLine(pt[0], pt[1]);
		}
		// ========================================================== //
		private function mouseJoint(p1:Particle, p2:Particle, len:int):void
		{
			var dx:Number;
			var dy:Number;
			var d_len:Number;
			var diff:Number;
 
			dx = p2.xx-p1.xx;
			dy = p2.yy-p1.yy;
			d_len = Math.sqrt(dx*dx + dy*dy);
			diff = (d_len-len)/d_len;
			dx *= diff;
			dy *= diff;
 
			p2.xx -= dx;
			p2.yy -= dy;
		}
		// ========================================================== //
		private function ptJoint(p1:Particle, p2:Particle, len:int):void
		{
			var dx:Number;
			var dy:Number;
			var d_len:Number;
			var diff:Number;
 
			dx = p2.xx-p1.xx;
			dy = p2.yy-p1.yy;
			d_len = Math.sqrt(dx*dx + dy*dy);
			diff = (d_len-len)/d_len;
			dx *= 0.5*diff;
			dy *= 0.5*diff;
 
			p2.xx -= dx;
			p2.yy -= dy;
			p1.xx += dx;
			p1.yy += dy;
		}
		// ========================================================== //
		private function drawLine(p1:Particle, p2:Particle):void
		{
			canvas.graphics.lineStyle(1,0x000000);
			canvas.graphics.moveTo(p1.x, p1.y);
			canvas.graphics.lineTo(p2.x, p2.y);
		}
		// ========================================================== //
	}
}
 
// ***** [ Particle Class ] ***************************************** //
 
import flash.display.Sprite;
 
internal class Particle extends Sprite
{
	public var old_x:Number;
	public var old_y:Number;
	public var xx:Number;
	public var yy:Number;
 
	// ========================================================== //
	public function Particle(_x:int, _y:int):void
	{
		x = _x;
		y = _y;
 
		xx = _x;
		yy = _y;
 
		old_x = _x;
		old_y = _y;
 
		graphics.lineStyle(1, 0x000000);
		graphics.beginFill(0xFFD22B);
		graphics.drawCircle(0,0,5);
		graphics.endFill();
	}
	// ========================================================== //
}