Diagnosing Godot RigidBody2D/3D That Won’t Move After Applying Force

Recognizable Condition

You apply a force or impulse to a RigidBody2D or RigidBody3D node (e.g., apply_impulse(Vector2.ZERO, Vector2(100,0))) but the body remains stationary in the game window.

Cause/Diagnostic Table

Possible Cause	What to Look For
Body mode set to Static or Kinematic	Forces are ignored; the body behaves like a fixed object.
Body is sleeping	Godot deactivates idle rigid bodies; forces have no effect until the body is woken.
Missing or disabled collision shape	Without a shape the physics engine cannot compute interactions, making impulses ineffective.
Custom physics integrator or Character mode	Alternative integration overrides default force handling.
Zero or near‑zero scale on the node	Extreme scale leads to infinite or NaN inertia, causing the engine to treat the body as immovable.

Ordered Checks

1. Verify the body's mode. In the inspector or via code, ensure mode is Rigid (value 0). If it is Static (1) or Kinematic (2), change it to Rigid. Note: Changing mode at runtime can affect existing joints; test in a copy of the scene first.
2. Check sleep state. Add a debug print in _physics_process: print($RigidBody.mode, $RigidBody.is_sleeping()). If the second value is true, the body is sleeping. Apply a small impulse or set wake_up() to see if it moves.
3. Confirm collision shape presence. Enable visible collision shapes (Project Settings → Physics → Common → Debug Shapes) or look at the node tree for a CollisionShape2D/CollisionShape3D child. Ensure its disabled property is false and that the shape resource is not null.
4. Inspect scale. Look at the node’s scale property. If any axis is close to zero (e.g., 0.0001) or negative large values, reset to Vector2.ONE or Vector3.ONE and test again.
5. Look for custom integrators. Check if a script overrides _integrate_forces or if the body’s custom_integrator property is set. Temporarily disable the script or set custom_integrator to false to see if default force handling resumes.

Fixes Tied to Findings

• If mode was wrong: set RigidBody.mode = RigidBody.MODE_RIGID.
• If sleeping: call RigidBody.wake_up() before applying force, or disable sleep for debugging via RigidBody.set_sleeping(false) (remember to re‑enable later).
• If missing shape: add a CollisionShape2D/CollisionShape3D child, assign a shape resource (e.g., RectangleShape2D), and ensure disabled is false.
• If scale issue: reset scale to Vector2.ONE or Vector3.ONE; avoid zero or extremely small values.
• If custom integrator: remove or comment out _integrate_forces override, or set custom_integrator to false in inspector.

Escalation Criteria

If after completing the five checks the body still does not move:

• Verify that no other script is constantly resetting the body’s position or velocity (e.g., in _process you set position = Vector2.ZERO).
• Test the same impulse in a fresh, minimal scene containing only the RigidBody, a collision shape, and a static floor. If it works there, the issue lies in the original scene’s node hierarchy or settings.
• Consider enabling verbose physics debug output (Project Settings → Physics → Common → Verbose) to look for warnings about invalid inertia or NaN values.
• If the problem persists, report it as a potential engine bug, providing the Godot version, a minimal reproduction project, and the exact steps taken.