title: Box2D - Falling body not accelerating under gravity
body: <p>I have created a basic example of a falling ball but I am slightly confused to why the object is not accelerating while falling.  It is travelling at constant speed which isn't what I would expect.  This is my first day using Box2D I assume I have missed something basic, but can't figure it out.</p>
  
  <pre><code>public PhysicsWorld() {
      // Step 1: Create Physics World Boundaries
      Vec2 gravity = new Vec2(0, 20);
      boolean doSleep = true;
      world = new World(gravity, doSleep);
  
      // Dynamic Body
      BodyDef bodyDef = new BodyDef();
      bodyDef.type = BodyType.DYNAMIC;
      bodyDef.position.set(100, 100);
      body = world.createBody(bodyDef);
      MassData md = new MassData();
      md.mass = 5;
      body.setMassData(md);
      PolygonShape dynamicBox = new PolygonShape();
      dynamicBox.setAsBox(1, 1);
      FixtureDef fixtureDef = new FixtureDef();
      fixtureDef.shape = dynamicBox;
      fixtureDef.density = 1;
      fixtureDef.friction = 0.3f;
      body.createFixture(fixtureDef);
  
      velocityIterations = 6;
      positionIterations = 2;
  
  }
  
  public void update() {
      world.step(timeStep, velocityIterations, positionIterations);
      Log.i("body", "x: " + body.getPosition().x + " y: " + body.getPosition().y);
  }
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>01-22 21:17:20.750: I/body(7698): x: 100.0 y: 102.0
  01-22 21:17:20.777: I/body(7698): x: 100.0 y: 104.0
  01-22 21:17:20.796: I/body(7698): x: 100.0 y: 106.0
  01-22 21:17:20.824: I/body(7698): x: 100.0 y: 108.0
  01-22 21:17:20.847: I/body(7698): x: 100.0 y: 110.0
  </code></pre>
  
  <p>I would expect gravity to be applied each iteration and increase the balls speed in Y.</p>