diff --git a/panda/src/bullet/bulletRigidBodyNode.cxx b/panda/src/bullet/bulletRigidBodyNode.cxx
index 8ff2e909b3..b76f6b9dcd 100644
--- a/panda/src/bullet/bulletRigidBodyNode.cxx
+++ b/panda/src/bullet/bulletRigidBodyNode.cxx
@@ -104,6 +104,7 @@ set_mass(float mass) {
   }
 
   _rigid->setMassProps(mass, inertia);
+  _rigid->updateInertiaTensor();
 }
 
 ////////////////////////////////////////////////////////////////////
diff --git a/panda/src/bullet/bulletSphericalConstraint.h b/panda/src/bullet/bulletSphericalConstraint.h
index 8b6b68afda..3b4efe1d30 100644
--- a/panda/src/bullet/bulletSphericalConstraint.h
+++ b/panda/src/bullet/bulletSphericalConstraint.h
@@ -27,7 +27,13 @@ class BulletRigidBodyNode;
 
 ////////////////////////////////////////////////////////////////////
 //       Class : BulletSphericalConstraint
-// Description : 
+// Description : A constraint between two rigid bodies, each with a
+//               pivot point. The pivot points are described in the
+//               body's local space. The constraint limits movement
+//               of the two rigid bodies in such a way that the 
+//               pivot points match in global space. The spherical
+//               constraint can be seen as a "ball and socket"
+//               joint.
 ////////////////////////////////////////////////////////////////////
 class EXPCL_PANDABULLET BulletSphericalConstraint : public BulletConstraint {