The Gravity in Box2D world - c++

Question

When the gravity is very small, the dynamic object will not move after moving for a short distance. When I run this method, the result is The change of dynamic body' position. When the dynamic body moves to a certain position, it will stop. I do not know why. Maybe the precision problem of floating number?
This is the method I run.
void initializeWorld()
{
// Define the gravity vector.
b2Vec2 gravity(0.0f, 0.011f);
// Construct a world object, which will hold and simulate the rigid bodies.
b2World world(gravity);
// Define the up ground body.
b2BodyDef groundBodyDefUp;
groundBodyDefUp.position.Set(0.0f, 8.0f);
// Define the down ground body.
b2BodyDef groundBodyDefDown;
groundBodyDefDown.position.Set(0.0f, -8.0f);
// Define the left ground body
b2BodyDef groundBodyDefLeft;
groundBodyDefLeft.position.Set(-8.0f,0.0f);
// Define the right ground body
b2BodyDef groundBodyDefRight;
groundBodyDefRight.position.Set(8.0f, 0.0f);
// Call the body factory which allocates memory for the ground body
// from a pool and creates the ground box shape (also from a pool).
// The body is also added to the world.
b2Body* groundBodyUp = world.CreateBody(&groundBodyDefUp);
b2Body* groundBodyDown = world.CreateBody(&groundBodyDefDown);
b2Body* groundBodyLeft = world.CreateBody(&groundBodyDefLeft);
b2Body* groundBodyRight = world.CreateBody(&groundBodyDefRight);
// Define the ground box shape.
b2PolygonShape groundBoxUp;
// The extents are the half-widths of the box.
groundBoxUp.SetAsBox(5.0f, 3.0f);
// Define the ground box shape.
b2PolygonShape groundBoxDown;
// The extents are the half-widths of the box.
groundBoxDown.SetAsBox(5.0f, 3.0f); // Define the ground box shape.
// Define the ground box shape.
b2PolygonShape groundBoxLeft;
// The extents are the half-widths of the box.
groundBoxLeft.SetAsBox(3.0f, 5.0f); // Define the ground box shape.
// Define the ground box shape.
b2PolygonShape groundBoxRight;
// The extents are the half-widths of the box.
groundBoxRight.SetAsBox(3.0f, 5.0f); // Define the ground box shape.
// Add the ground fixture to the ground body.
groundBodyUp->CreateFixture(&groundBoxUp, 0.0f);
/*groundBodyDown->CreateFixture(&groundBoxDown, 0.0f);
groundBodyLeft->CreateFixture(&groundBoxLeft, 0.0f);
groundBodyRight->CreateFixture(&groundBoxRight, 0.0f);*/
// Define the dynamic body. We set its position and call the body factory.
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;
bodyDef.position.Set(0.0f, 0.0f);
b2Body* body = world.CreateBody(&bodyDef);
// Define another box shape for our dynamic body.
b2PolygonShape dynamicBox;
dynamicBox.SetAsBox(0.5f, 0.5f);
// Define the dynamic body fixture.
b2FixtureDef fixtureDef;
fixtureDef.shape = &dynamicBox;
// Set the box density to be non-zero, so it will be dynamic.
fixtureDef.density = 1.0f;
// Override the default friction.
fixtureDef.friction = 0.3f;
// Add the shape to the body.
body->CreateFixture(&fixtureDef);
// Prepare for simulation. Typically we use a time step of 1/60 of a
// second (60Hz) and 10 iterations. This provides a high quality simulation
// in most game scenarios.
float32 timeStep = 1.0f / 60.0f;
int32 velocityIterations = 6;
int32 positionIterations = 2;
bool running = true;
while (running)
{
world.Step(timeStep, velocityIterations, positionIterations);
b2Vec2 position = body->GetPosition();
cout << "(" << position.x << "," << position.y <<")"<< endl;
}
}