diff --git a/Engine.h b/Engine.h
index ab0287f..47bdaf5 100644
--- a/Engine.h
+++ b/Engine.h
@@ -140,6 +140,11 @@ class Engine
         bool importFullPath(const char * path);
         GLuint loadTexture(const char * name);
         void debug_hook(lua_Debug * debug);
+        void setGravity(float gx, float gy, float gz)
+        {
+            m_world.setGravity(gx, gy, gz);
+        }
+
         void getScreenSize(int * width, int * height);
         void drawText(const char * text, GLfloat r, GLfloat g, GLfloat b,
                 int ptsize, float x, float y);
diff --git a/ag.cc b/ag.cc
index a109a02..52c7fb7 100644
--- a/ag.cc
+++ b/ag.cc
@@ -50,6 +50,7 @@ namespace ag
             { "setAutoPhysics", setAutoPhysics },
             { "setAutoStartFrame", setAutoStartFrame },
             { "setCamera", setCamera },
+            { "setGravity", setGravity },
 //            { "sleep", sleep },
             { "startFrame", startFrame },
 
@@ -168,6 +169,10 @@ namespace ag
         lua_setfield(L, -2, "setMass");
         lua_pushcfunction(L, object::getMass);
         lua_setfield(L, -2, "getMass");
+        lua_pushcfunction(L, object::getAABB);
+        lua_setfield(L, -2, "getAABB");
+        lua_pushcfunction(L, object::getSize);
+        lua_setfield(L, -2, "getSize");
     }
 
     static int loadModelSpecify(lua_State * L, bool static_data)
@@ -241,6 +246,21 @@ namespace ag
         return g_engine->setCamera(L);
     }
 
+    int setGravity(lua_State * L)
+    {
+        int argc = lua_gettop(L);
+        if (argc == 3
+                && lua_isnumber(L, 1)
+                && lua_isnumber(L, 2)
+                && lua_isnumber(L, 3))
+        {
+            g_engine->setGravity(lua_tonumber(L, 1),
+                    lua_tonumber(L, 2),
+                    lua_tonumber(L, 3));
+        }
+        return 0;
+    }
+
     int getCamera(lua_State * L)
     {
         return g_engine->getCamera(L);
diff --git a/ag.h b/ag.h
index dd6d290..1ced059 100644
--- a/ag.h
+++ b/ag.h
@@ -30,6 +30,7 @@ namespace ag
     int setAutoPhysics(lua_State * L);
     int setAutoStartFrame(lua_State * L);
     int setCamera(lua_State * L);
+    int setGravity(lua_State * L);
     int sleep(lua_State * L);
     int startFrame(lua_State * L);
 
diff --git a/doc/index.html b/doc/index.html
index 4a0e04b..9e73382 100644
--- a/doc/index.html
+++ b/doc/index.html
@@ -393,6 +393,13 @@ The default center point is (0, 0, 0).
 The default up vector is (0, 0, 1).
 </p>
 
+<a name="ag_setGravity" />
+<h3>setGravity</h3>
+<p><tt>ag.setGravity(gx, gy, gz)</tt></p>
+<p>
+This function sets the gravity vector to (gx, gy, gz).
+</p>
+
 <a name="ag_startFrame" />
 <h3>startFrame</h3>
 <p><tt>ag.startFrame()</tt></p>