Rename contactFrictionStiffness to contactFrictionGain

The kf parameter is a velocity-dependent friction gain, not a
tangential stiffness. Ordered with the other contact* attrs:
contactStiffness, contactDamping, contactFrictionGain, contactAdhesion.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: andrewkaufman

CHANGELOG.md

@@ -1,3 +1,9 @@
+# 0.3.1
+
+## Fixes
+
+- Renamed `newton:contactFrictionStiffness` to `newton:contactFrictionGain` on `NewtonMaterialAPI`; the parameter is a velocity-dependent friction gain, not a tangential stiffness
+
 # 0.3.0
 
 ## Features

newton_usd_schemas/_version.py

@@ -1,4 +1,4 @@
 # SPDX-FileCopyrightText: Copyright (c) 2025 The Newton Developers
 # SPDX-License-Identifier: Apache-2.0
 
-__version__ = "0.3.0"
+__version__ = "0.3.1"

newton_usd_schemas/generatedSchema.usda

@@ -691,13 +691,15 @@ class NewtonMaterialAPI "NewtonMaterialAPI" (
         }
     )
 
-    float newton:contactFrictionStiffness = -inf (
-        doc = """Stiffness of the tangential friction response.
+    float newton:contactFrictionGain = -inf (
+        doc = """Friction gain controlling resistance to tangential slip.
 
-        At low sliding speeds, friction force equals `contactFrictionStiffness * velocity`
-        (viscous regime); at high speeds it is clamped to the Coulomb limit `mu * normal_force`.
+        Controls the slope of the friction force with respect to tangential slip velocity
+        in the sticking regime. Larger values produce stronger resistance to small relative
+        motion and more closely approximate ideal Coulomb sticking. At high speeds the
+        friction force is clamped to the Coulomb limit `mu * normal_force`.
 
-        If `newton:contactFrictionStiffness` is set to `-inf`, the engine's default should be used instead.
+        If `newton:contactFrictionGain` is set to `-inf`, the engine's default should be used instead.
 
         Range: [0, inf)
         Units: force * time / distance"""

tests/test_material.py

@@ -33,7 +33,7 @@ def test_api_application(self):
         self.assertTrue(self.material.HasAttribute("newton:torsionalFriction"))  # from NewtonMaterialAPI
         self.assertTrue(self.material.HasAttribute("newton:contactStiffness"))  # from NewtonMaterialAPI
         self.assertTrue(self.material.HasAttribute("newton:contactDamping"))  # from NewtonMaterialAPI
-        self.assertTrue(self.material.HasAttribute("newton:contactFrictionStiffness"))  # from NewtonMaterialAPI
+        self.assertTrue(self.material.HasAttribute("newton:contactFrictionGain"))  # from NewtonMaterialAPI
         self.assertTrue(self.material.HasAttribute("newton:contactAdhesion"))  # from NewtonMaterialAPI
 
     def test_api_limitations(self):
@@ -121,10 +121,10 @@ def test_contact_damping(self):
             self.assertIsNone(soft.GetMaximum())
 
     def test_contact_friction_stiffness(self):
-        self.assertFalse(self.material.HasAttribute("newton:contactFrictionStiffness"))
+        self.assertFalse(self.material.HasAttribute("newton:contactFrictionGain"))
 
         self.material.ApplyAPI("NewtonMaterialAPI")
-        attr = self.material.GetAttribute("newton:contactFrictionStiffness")
+        attr = self.material.GetAttribute("newton:contactFrictionGain")
         self.assertIsNotNone(attr)
         self.assertFalse(attr.HasAuthoredValue())
         self.assertEqual(attr.Get(), -math.inf)