jeffreya12_SSN/Apuntes/sketch_9_26_2017/ejemploRevoluteJoint/Propeller.pde
class Propeller {
  PolygonParticle spike;
  Box base;
  Joint joint;
  float w = 10;
  float h = 180;
  
  Propeller(float x, float y) {
    base = new Box(x, y - h/2, w, h);
    spike = new PolygonParticle(x, y - h + w/2);
    
    RevoluteJointDef rjd = new RevoluteJointDef();
    rjd.initialize(base.body, spike.body, spike.body.getTransform().p);
    rjd.motorSpeed = PI * 1;
    rjd.maxMotorTorque = 50000;
    rjd.enableMotor = true;
    
    joint = box2d.world.createJoint(rjd); 
  }
  void display() {
    base.display();
    spike.display();
  }
}