diff --git a/GameCli.Tests/PhysicsTests.cs b/GameCli.Tests/PhysicsTests.cs
new file mode 100644
index 0000000..9e2ec9a
--- /dev/null
+++ b/GameCli.Tests/PhysicsTests.cs
@@ -0,0 +1,64 @@
+using GameCli;
+using Xunit;
+
+namespace GameCli.Tests;
+
+public class PhysicsTests
+{
+    [Fact]
+    public void Noop_UnderGravity_ShipFallsDownward()
+    {
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 0);
+        Assert.True(next.VY > 0, $"expected VY > 0 (gravity down), got {next.VY}");
+        Assert.True(next.Y > start.Y, $"expected Y to increase (fall), got {next.Y}");
+        Assert.Equal(start.X, next.X, precision: 5);
+        Assert.Equal(0f, next.Angle);
+    }
+
+    [Fact]
+    public void MainEngine_WhenUpright_CancelsThenReversesGravity()
+    {
+        // Main engine thrust > gravity, so acceleration is net upward (VY becomes negative).
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 2);
+        Assert.True(next.VY < 0, $"expected VY < 0 (net upward), got {next.VY}");
+    }
+
+    [Fact]
+    public void LeftThruster_AppliesNegativeTorque()
+    {
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 1);
+        Assert.True(next.AngularVelocity < 0,
+            $"expected angular velocity < 0, got {next.AngularVelocity}");
+    }
+
+    [Fact]
+    public void RightThruster_AppliesPositiveTorque()
+    {
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var next = Physics.Step(start, action: 3);
+        Assert.True(next.AngularVelocity > 0,
+            $"expected angular velocity > 0, got {next.AngularVelocity}");
+    }
+
+    [Fact]
+    public void MainEngine_WhenRotated90Right_ThrustsRight()
+    {
+        // Angle = +pi/2 means ship points to the +x direction (right).
+        // Main engine pushes along body-up, which is now world +x.
+        var start = new ShipState(0.5f, 0.5f, 0f, 0f, MathF.PI / 2f, 0f);
+        var next = Physics.Step(start, action: 2);
+        Assert.True(next.VX > 0, $"expected VX > 0 (thrust right), got {next.VX}");
+    }
+
+    [Fact]
+    public void Step_IsDeterministic()
+    {
+        var start = new ShipState(0.3f, 0.4f, 0.1f, -0.05f, 0.2f, 0.3f);
+        var a = Physics.Step(start, action: 2);
+        var b = Physics.Step(start, action: 2);
+        Assert.Equal(a, b);
+    }
+}
diff --git a/GameCli/Physics.cs b/GameCli/Physics.cs
new file mode 100644
index 0000000..d1f0477
--- /dev/null
+++ b/GameCli/Physics.cs
@@ -0,0 +1,78 @@
+namespace GameCli;
+
+/// <summary>
+/// Pure physics stepper. All constants are public so training/tuning can inspect them.
+/// Explicit Euler integration at fixed dt.
+/// </summary>
+public static class Physics
+{
+    public const float Dt = 0.02f;                 // 50 Hz
+    public const float Gravity = 0.5f;             // world units / s^2, +Y (downward)
+    public const float MainThrust = 1.2f;          // world units / s^2 along body-up
+    public const float SideTorque = 4.0f;          // rad / s^2
+    public const float SideLateralImpulse = 0.15f; // world units / s^2 along body-x
+    public const float LinearDrag = 0.10f;         // per-second
+    public const float AngularDrag = 0.50f;        // per-second
+
+    public const int ActionNoop = 0;
+    public const int ActionLeft = 1;
+    public const int ActionMain = 2;
+    public const int ActionRight = 3;
+
+    /// <summary>
+    /// Advance the ship state by one dt given a discrete action.
+    /// Gravity is always applied. Thrusters add to acceleration/torque.
+    /// </summary>
+    public static ShipState Step(ShipState s, int action)
+    {
+        // Start with gravity.
+        float ax = 0f;
+        float ay = Gravity;
+        float torque = 0f;
+
+        // Body-up direction in world coords, given angle:
+        //   angle=0    → (0, -1)    "up" on screen (Y is down)
+        //   angle=+π/2 → (+1,  0)   right
+        //   angle=+π   → (0, +1)    down
+        float sinA = MathF.Sin(s.Angle);
+        float cosA = MathF.Cos(s.Angle);
+        float bodyUpX = sinA;
+        float bodyUpY = -cosA;
+        // Body-right direction (perpendicular, rotated +90°):
+        float bodyRightX = cosA;
+        float bodyRightY = sinA;
+
+        switch (action)
+        {
+            case ActionMain:
+                ax += bodyUpX * MainThrust;
+                ay += bodyUpY * MainThrust;
+                break;
+            case ActionLeft:
+                torque -= SideTorque;
+                ax += bodyRightX * SideLateralImpulse;
+                ay += bodyRightY * SideLateralImpulse;
+                break;
+            case ActionRight:
+                torque += SideTorque;
+                ax -= bodyRightX * SideLateralImpulse;
+                ay -= bodyRightY * SideLateralImpulse;
+                break;
+            case ActionNoop:
+            default:
+                break;
+        }
+
+        // Integrate velocity, apply linear drag, integrate position.
+        float vx = (s.VX + ax * Dt) * (1f - LinearDrag * Dt);
+        float vy = (s.VY + ay * Dt) * (1f - LinearDrag * Dt);
+        float x = s.X + vx * Dt;
+        float y = s.Y + vy * Dt;
+
+        // Angular: same pattern.
+        float w = (s.AngularVelocity + torque * Dt) * (1f - AngularDrag * Dt);
+        float angle = s.Angle + w * Dt;
+
+        return new ShipState(x, y, vx, vy, angle, w);
+    }
+}