Attempting to make a bouncing ball with Cocos2D XNA and Box2D

IntroLayer.cs

using System;

using Cocos2D;

using Box2D.Dynamics;
using Box2D.Common;
using Box2D.Collision.Shapes;

using Microsoft.Xna.Framework;

namespace Cocos2DGame1
{
    public class IntroLayer : CCLayerColor
    {
		float PtmRatio = 32f;

		b2World World;

		CCSize WinSize { get { return CCDirector.SharedDirector.WinSize; } }

        public IntroLayer()
        {
			var ballSprite = new CCSprite ("ball.png", new CCRect (0, 0, 80, 80));
			ballSprite.Position = new CCPoint (WinSize.Center.X, WinSize.Height);
			AddChild (ballSprite);

			var gravity = new b2Vec2 (0.0f, -9.8f);
			World = new b2World (gravity);
			World.SetContinuousPhysics (true);

			var ballBodyDef = b2BodyDef.Create ();
			ballBodyDef.userData = ballSprite;
			ballBodyDef.type = b2BodyType.b2_dynamicBody;
			ballBodyDef.position.Set (ballSprite.PositionX / PtmRatio,
			                          ballSprite.PositionY / PtmRatio);

			var ballBody = World.CreateBody (ballBodyDef);

			var circle = new b2CircleShape ();
			circle.Radius = 80f / PtmRatio;

			var ballShapeDef = new b2FixtureDef ();
			ballShapeDef.shape = circle;
			ballShapeDef.density = 1.0f;
			ballShapeDef.friction = 0.2f;
			ballShapeDef.restitution = 0.8f;

			ballBody.CreateFixture (ballShapeDef);

			var groundBodyDef = b2BodyDef.Create();
			groundBodyDef.position.Set (0, 0);

			var groundBody = World.CreateBody (groundBodyDef);
			var groundEdge = new b2EdgeShape ();

			groundEdge.Set (new b2Vec2 (0, 0), new b2Vec2 (WinSize.Width / PtmRatio, 0));
			groundBody.CreateFixture (groundEdge, 0);
        }

		void Step(float dt)
		{
			World.Step (dt, 5, 1);

			for (var body = World.BodyList; body != null; body = body.Next) {
				if (body.UserData != null) {
					var ball = body.UserData as CCSprite;
					ball.PositionX = body.Position.x * PtmRatio;
					ball.PositionY = body.Position.y * PtmRatio;
					ball.Rotation = -1 * CCMacros.CCRadiansToDegrees (body.Angle);
				}
			}
		}

		public override void OnEnter ()
		{
			base.OnEnter ();
			Schedule (Step);
		}

        public static CCScene Scene
        {
            get
            {
                var scene = new CCScene();
				var layer = new IntroLayer();

				scene.AddChild(layer);
                return scene;
            }
        }
    }
}