Merge pull request #157 from derek-mcblane/simple-frictional-inelastic

Fix Error in Ball-Ball Frictional Inelastic No-Slip Velocity Calculation

pooltool/physics/resolve/ball_ball/frictional_inelastic/__init__.py

@@ -71,13 +71,12 @@ def _resolve_ball_ball(rvw1, rvw2, R, u_b, e_b):
     # then slip condition is invalid so we need to calculate no-slip condition
     if not has_relative_velocity or np.dot(v12_c, v12_c_slip) <= 0:  # type: ignore
         # velocity tangent component for no-slip condition
-        D_v1_t = (
-            2.0 * rvw2[1] - R * ptmath.cross(2.0 * rvw1[2] + 7.0 * rvw2[2], unit_x)
-        ) / 9.0
-        D_w1 = (
-            5.0
-            * (rvw2[2] - rvw1[2] + ptmath.cross(unit_x, rvw2[1] - rvw1[1]) / R)
-            / 9.0
+        D_v1_t = -(1.0 / 9.0) * (
+            2.0 * (rvw1[1] - rvw2[1])
+            + R * ptmath.cross(2.0 * rvw1[2] + 7.0 * rvw2[2], unit_x)
+        )
+        D_w1 = (5.0 / 9.0) * (
+            rvw2[2] - rvw1[2] + ptmath.cross(unit_x, rvw2[1] - rvw1[1]) / R
         )
         rvw1_f[1] = rvw1[1] + D_v1_t
         rvw1_f[2] = rvw1[2] + D_w1

tests/physics/resolve/ball_ball/test_ball_ball.py

@@ -23,6 +23,19 @@ def head_on() -> Tuple[Ball, Ball]:
     return cb, ob
 
 
+def translating_head_on() -> Tuple[Ball, Ball]:
+    cb = Ball.create("cue", xy=(0, 0))
+    ob = Ball.create("cue", xy=(2 * cb.params.R, 0))
+
+    # Cue ball makes head-on collision with object ball at 1 m/s in +x direction
+    # while both balls move together at 1 m/s in +y direction
+    cb.state.rvw[1] = np.array([1, 1, 0])
+    ob.state.rvw[1] = np.array([0, 1, 0])
+
+    assert cb.params.m == ob.params.m, "Balls expected to be equal mass"
+    return cb, ob
+
+
 @pytest.mark.parametrize("model", [FrictionlessElastic()])
 def test_head_on_zero_spin(model: BallBallCollisionStrategy):
     cb_i, ob_i = head_on()
@@ -63,6 +76,23 @@ def test_head_on_zero_spin_inelastic(model: BallBallCollisionStrategy, e_b: floa
         assert cb_f.state.rvw[1][0] > 0
 
 
+@pytest.mark.parametrize("model", [FrictionalInelastic(), FrictionalMathavan()])
+@pytest.mark.parametrize("e_b", [0.5, 0.6, 0.7, 0.8, 0.9, 1.0])
+def test_translating_head_on_zero_spin_inelastic(
+    model: BallBallCollisionStrategy, e_b: float
+):
+    cb_i, ob_i = translating_head_on()
+
+    # Update coefficient of restitutions
+    cb_i.params = attrs.evolve(cb_i.params, e_b=e_b)
+    ob_i.params = attrs.evolve(ob_i.params, e_b=e_b)
+
+    cb_f, ob_f = model.resolve(cb_i, ob_i, inplace=False)
+
+    # Balls should still be moving together in +y direction
+    assert np.isclose(cb_f.vel[1], ob_f.vel[1], atol=1e-10)
+
+
 @pytest.mark.parametrize("model", [FrictionalInelastic(), FrictionalMathavan()])
 @pytest.mark.parametrize("cb_wz_i", [0.1, 1, 10, 100])
 def test_head_on_z_spin(model: BallBallCollisionStrategy, cb_wz_i: float):