diff --git a/Engine.cc b/Engine.cc
index ee0b75e..066831f 100644
--- a/Engine.cc
+++ b/Engine.cc
@@ -1101,6 +1101,9 @@ Engine::Object::Object(bool is_static, bool is_reference,
     m_scale = scale;
     m_is_scaled = ! (fabs(scale - 1.0) < 0.0001);
     m_is_managed = false;
+    m_mass = 0.0;
+    m_mass_is_set = false;
+    m_gravity_mode = true;
 }
 
 /* used for "managed" objects with one geom */
@@ -1133,6 +1136,9 @@ Engine::Object::Object(bool is_static, bool is_reference, OdeWorld & world,
     m_texture_scale = 1.0f;
     createManagedObject();
     render();
+    m_mass = 0.0;
+    m_mass_is_set = false;
+    m_gravity_mode = true;
 }
 
 /* used to clone objects */
@@ -1173,6 +1179,14 @@ Engine::Object::Object(const Engine::Object & orig)
         createManagedObject();
         render();
     }
+    m_mass = orig.m_mass;
+    m_mass_is_set = orig.m_mass_is_set;
+    if (m_mass_is_set)
+    {
+        setMass(orig.getMass());
+    }
+    m_gravity_mode = orig.m_gravity_mode;
+    setGravityMode(m_gravity_mode);
 }
 
 Engine::Object::~Object()
@@ -1459,6 +1473,20 @@ void Engine::Object::render()
     checkGLError();
 }
 
+void Engine::Object::setPosition(double x, double y, double z)
+{
+    if (m_ode_object != NULL)
+        m_ode_object->setPosition(x, y, z);
+}
+
+void Engine::Object::getPosition(double * x, double * y, double * z)
+{
+    if (m_ode_object != NULL)
+        m_ode_object->getPosition(x, y, z);
+    else
+        *x = *y = *z = 0.0;
+}
+
 void Engine::Object::loadPhy(FileLoader * fl, const FileLoader::Path & path)
 {
     m_phy = new PhyObj();
@@ -1542,6 +1570,21 @@ void Engine::Object::draw()
     }
 }
 
+dReal Engine::Object::getMass() const
+{
+    return (m_ode_object != NULL) ? m_ode_object->getMass() : m_mass;
+}
+
+void Engine::Object::setMass(dReal mass)
+{
+    m_mass = mass;
+    m_mass_is_set = true;
+    if (m_ode_object != NULL)
+    {
+        m_ode_object->setMass(mass);
+    }
+}
+
 
 /******** Engine::EngineFileLoader functions ********/
 
diff --git a/Engine.h b/Engine.h
index ee0c108..e0679cc 100644
--- a/Engine.h
+++ b/Engine.h
@@ -30,37 +30,36 @@ class Engine
                 Object(const Object & orig);
                 ~Object();
 
-                void setPosition(double x, double y, double z)
-                {
-                    m_ode_object->setPosition(x, y, z);
-                }
-                void getPosition(double * x, double * y, double * z)
-                {
-                    m_ode_object->getPosition(x, y, z);
-                }
+                void setPosition(double x, double y, double z);
+                void getPosition(double * x, double * y, double * z);
                 void loadPhy(FileLoader * fl, const FileLoader::Path & path);
                 void instantiatePhy();
                 void setVisible(bool visible) { m_is_visible = visible; }
                 bool getVisible() { return m_is_visible; }
                 void addForce(dReal fx, dReal fy, dReal fz)
                 {
-                    m_ode_object->addForce(fx, fy, fz);
+                    if (m_ode_object != NULL)
+                        m_ode_object->addForce(fx, fy, fz);
                 }
                 void addForceRel(dReal fx, dReal fy, dReal fz)
                 {
-                    m_ode_object->addForceRel(fx, fy, fz);
+                    if (m_ode_object != NULL)
+                        m_ode_object->addForceRel(fx, fy, fz);
                 }
                 void addTorque(dReal fx, dReal fy, dReal fz)
                 {
-                    m_ode_object->addTorque(fx, fy, fz);
+                    if (m_ode_object != NULL)
+                        m_ode_object->addTorque(fx, fy, fz);
                 }
                 void addTorqueRel(dReal fx, dReal fy, dReal fz)
                 {
-                    m_ode_object->addTorqueRel(fx, fy, fz);
+                    if (m_ode_object != NULL)
+                        m_ode_object->addTorqueRel(fx, fy, fz);
                 }
                 void setRotation(dReal x, dReal y, dReal z)
                 {
-                    m_ode_object->setRotation(x, y, z);
+                    if (m_ode_object != NULL)
+                        m_ode_object->setRotation(x, y, z);
                 }
                 void setColor(float r, float g, float b)
                 {
@@ -77,13 +76,18 @@ class Engine
                     render();
                 }
                 void draw();
-                dReal getMass() { return m_ode_object->getMass(); }
-                void setMass(dReal mass) { m_ode_object->setMass(mass); }
+                dReal getMass() const;
+                void setMass(dReal mass);
                 const float * getAABB() { return m_aabb; }
-                dBodyID getBody() { return m_ode_object->getBody(); }
+                dBodyID getBody()
+                {
+                    return (m_ode_object != NULL) ? m_ode_object->getBody() : 0;
+                }
                 void setGravityMode(bool enabled)
                 {
-                    m_ode_object->setGravityMode(enabled);
+                    m_gravity_mode = enabled;
+                    if (m_ode_object != NULL)
+                        m_ode_object->setGravityMode(enabled);
                 }
 
             protected:
@@ -101,6 +105,9 @@ class Engine
                 float m_aabb[6];
                 OdeWorld & m_world;
                 refptr<PhyObj> m_phy;
+                dReal m_mass;
+                bool m_mass_is_set;
+                bool m_gravity_mode;
 
                 /* for "pre-loaded" objects */
                 int * m_display_list_refcnt;
diff --git a/lib/demo/bowling.lua b/lib/demo/bowling.lua
index 7b5d52e..39db81e 100644
--- a/lib/demo/bowling.lua
+++ b/lib/demo/bowling.lua
@@ -68,18 +68,14 @@ function init_event()
     local lane_bottom = minz
     local lane_top = maxz
     pins = {}
-    reference_pin = ag.loadModel("bowling_pin", {scale = 1.0/4.3})
+    reference_pin = ag.loadModel("bowling_pin",
+        {scale = 1.0/4.3, reference = true})
     minx, miny, minz, maxx, maxy, maxz = reference_pin:getAABB()
     pin_minz = minz
     pin_maxz = maxz
     pin_standing_position = lane_top - pin_minz
-    local box = ag.createBox(1, 10, 0.1, {static = true})
-    box:setVisible(false)
-    box:setPosition(0, 0, lane_bottom - (maxz - minz) - 0.4)
-    reference_pin:setPosition(0, 0, lane_bottom - maxz - 0.1)
-    reference_ball = ag.loadModel("bowling_ball")
+    reference_ball = ag.loadModel("bowling_ball", {reference = true})
     reference_ball:setMass(10)
-    reference_ball:setPosition(0, -2, -1.5)
     setupStars()
     game_state = "waiting"
     mouse_moved = false