demos.crank = {};
  demos.crank.initWorld = function(world) {
          var ground = world.m_groundBody;
  
          // Define crank.
          var sd = new b2BoxDef();
          sd.extents.Set(5, 25);
          sd.density = 1.0;
  
          var bd = new b2BodyDef();
          bd.AddShape(sd);
          
          var rjd = new b2RevoluteJointDef();
  
          var prevBody = ground;
  
          bd.position.Set(500/2, 210);
          var body = world.CreateBody(bd);
  
          rjd.anchorPoint.Set(500/2, 235);
          rjd.body1 = prevBody;
          rjd.body2 = body;
          rjd.motorSpeed = -1.0 * Math.PI;
          rjd.motorTorque = 500000000.0;
          rjd.enableMotor = true;
          world.CreateJoint(rjd);
  
          prevBody = body;
  
          // Define follower.
          sd.extents.Set(5, 45);
          bd.position.Set(500/2, 140);
          body = world.CreateBody(bd);
  
          rjd.anchorPoint.Set(500/2, 185);
          rjd.body1 = prevBody;
          rjd.body2 = body;
          rjd.enableMotor = false;
          world.CreateJoint(rjd);
  
          prevBody = body;
  
          // Define piston
          sd.extents.Set(20, 20);
          bd.position.Set(500/2, 95);
          body = world.CreateBody(bd);
  
          rjd.anchorPoint.Set(500/2, 95);
          rjd.body1 = prevBody;
          rjd.body2 = body;
          world.CreateJoint(rjd);
  
          var pjd = new b2PrismaticJointDef();
          pjd.anchorPoint.Set(500/2, 95);
          pjd.body1 = ground;
          pjd.body2 = body;
          pjd.axis.Set(0.0, 1.0);
          pjd.motorSpeed = 0.0; // joint friction
          pjd.motorForce = 100000.0;
          pjd.enableMotor = true;
  
          world.CreateJoint(pjd);
  
          // Create a payload
          sd.density = 2.0;
          bd.position.Set(500/2, 10);
          world.CreateBody(bd);
  }
  demos.InitWorlds.push(demos.crank.initWorld);