From 4940ea6dd700fc6505b40a7cf6b38b5895120a11 Mon Sep 17 00:00:00 2001
From: Jeremy Nimmer <jeremy.nimmer@tri.global>
Date: Wed, 27 Mar 2024 18:41:10 -0700
Subject: [PATCH] WIP try a default of zero

---
 multibody/plant/multibody_plant.h | 6 ++----
 multibody/tree/rigid_body.cc      | 4 +---
 multibody/tree/rigid_body.h       | 6 ++----
 3 files changed, 5 insertions(+), 11 deletions(-)

diff --git a/multibody/plant/multibody_plant.h b/multibody/plant/multibody_plant.h
index 5dfc07433845..fc94693add8a 100644
--- a/multibody/plant/multibody_plant.h
+++ b/multibody/plant/multibody_plant.h
@@ -1159,8 +1159,7 @@ class MultibodyPlant : public internal::MultibodyTreeSystem<T> {
   /// @param[in] M_BBo_B
   ///   The SpatialInertia of the new rigid body to be added to `this`
   ///   %MultibodyPlant, computed about the body frame origin `Bo` and expressed
-  ///   in the body frame B. When not provided, defaults to a nominal non-zero
-  ///   value.
+  ///   in the body frame B. When not provided, defaults to ...
   /// @returns A constant reference to the new RigidBody just added, which will
   ///          remain valid for the lifetime of `this` %MultibodyPlant.
   const RigidBody<T>& AddRigidBody(
@@ -1206,8 +1205,7 @@ class MultibodyPlant : public internal::MultibodyTreeSystem<T> {
   /// @param[in] M_BBo_B
   ///   The SpatialInertia of the new rigid body to be added to `this`
   ///   %MultibodyPlant, computed about the body frame origin `Bo` and expressed
-  ///   in the body frame B. When not provided, defaults to a nominal non-zero
-  ///   value.
+  ///   in the body frame B. When not provided, defaults to ...
   /// @returns A constant reference to the new RigidBody just added, which will
   ///          remain valid for the lifetime of `this` %MultibodyPlant.
   /// @throws std::exception if additional model instances have been created
diff --git a/multibody/tree/rigid_body.cc b/multibody/tree/rigid_body.cc
index 8a2c8f88431c..57cc1948d426 100644
--- a/multibody/tree/rigid_body.cc
+++ b/multibody/tree/rigid_body.cc
@@ -65,9 +65,7 @@ RigidBody<T>::RigidBody(const std::string& body_name,
 
 template <typename T>
 SpatialInertia<double> RigidBody<T>::MakeNominalSpatialInertiaForConstructor() {
-  const double mass = 1.0;
-  const double radius = 0.0625;
-  return SpatialInertia<double>::SolidSphereWithMass(mass, radius);
+  return SpatialInertia<double>::Zero();
 }
 
 template <typename T>
diff --git a/multibody/tree/rigid_body.h b/multibody/tree/rigid_body.h
index d2b1db5bc9cd..99332be3a7b3 100644
--- a/multibody/tree/rigid_body.h
+++ b/multibody/tree/rigid_body.h
@@ -197,8 +197,7 @@ class RigidBody : public MultibodyElement<T> {
   ///   A name associated with this body.
   /// @param[in] M_BBo_B
   ///   Spatial inertia of this body B about the frame's origin `Bo` and
-  ///   expressed in the body frame B. When not provided, defaults to a nominal
-  ///   non-zero value.
+  ///   expressed in the body frame B. When not provided, defaults to ...
   /// @note See @ref multibody_spatial_inertia for details on the monogram
   /// notation used for spatial inertia quantities.
   explicit RigidBody(const std::string& body_name,
@@ -214,8 +213,7 @@ class RigidBody : public MultibodyElement<T> {
   ///   The model instance associated with this body.
   /// @param[in] M_BBo_B
   ///   Spatial inertia of this body B about the frame's origin `Bo` and
-  ///   expressed in the body frame B. When not provided, defaults to a nominal
-  ///   non-zero value.
+  ///   expressed in the body frame B. When not provided, defaults to ...
   /// @note See @ref multibody_spatial_inertia for details on the monogram
   /// notation used for spatial inertia quantities.
   RigidBody(const std::string& body_name, ModelInstanceIndex model_instance,