Game Physics Simulator C#

```csharp
using System;
using System.Collections.Generic;
using UnityEngine;

public class PhysicsSimulator : MonoBehaviour
{
    [Header("General Settings")]
    public float gravity = -9.81f;
    public int integrationSteps = 10; // More steps, more accurate but slower

    [Header("Objects")]
    public List<SimulatedObject> simulatedObjects = new List<SimulatedObject>();

    private float fixedDeltaTime;

    void Start()
    {
        fixedDeltaTime = Time.fixedDeltaTime;
    }


    void FixedUpdate()
    {
        Simulate();
    }

    void Simulate()
    {
        float subDeltaTime = fixedDeltaTime / integrationSteps;

        for (int step = 0; step < integrationSteps; step++)
        {
            foreach (SimulatedObject obj in simulatedObjects)
            {
                if (obj != null && obj.usePhysics) // Check if object is valid and physics is enabled
                {
                    // 1. Calculate forces
                    Vector3 totalForce = CalculateForces(obj);

                    // 2. Calculate Acceleration
                    Vector3 acceleration = totalForce / obj.mass;

                    // 3. Integrate Velocity (Euler Integration)
                    obj.velocity += acceleration * subDeltaTime;

                    // 4. Integrate Position (Euler Integration)
                    obj.transform.position += obj.velocity * subDeltaTime;

                    // 5. Damping
                    obj.velocity *= (1f - obj.damping * subDeltaTime);


                    // 6.  Handle collisions.  This is simplified for brevity and assumes world boundaries.
                    HandleCollisions(obj);
                }
            }
        }
    }


    Vector3 CalculateForces(SimulatedObject obj)
    {
        Vector3 totalForce = Vector3.zero;

        // Apply gravity
        totalForce += Vector3.up * gravity * obj.mass;  // Using Vector3.up directly

        //Apply custom forces if necessary
        totalForce += obj.additionalForce;

        return totalForce;
    }

    void HandleCollisions(SimulatedObject obj)
    {
        // Simple floor collision.  Adapt as necessary.
        if (obj.transform.position.y < obj.floorY)
        {
            obj.transform.position = new Vector3(obj.transform.position.x, obj.floorY, obj.transform.position.z);
            obj.velocity.y = -obj.velocity.y * obj.restitution;  // Basic bounce

            // Apply friction on the XZ plane
            obj.velocity.x *= (1f - obj.friction);
            obj.velocity.z *= (1f - obj.friction);
        }

        // Simple wall collisions
        if (obj.transform.position.x < obj.wallMinX)
        {
            obj.transform.position = new Vector3(obj.wallMinX, obj.transform.position.y, obj.transform.position.z);
            obj.velocity.x = -obj.velocity.x * obj.restitution;
        }
        else if (obj.transform.position.x > obj.wallMaxX)
        {
             obj.transform.position = new Vector3(obj.wallMaxX, obj.transform.position.y, obj.transform.position.z);
            obj.velocity.x = -obj.velocity.x * obj.restitution;
        }

         if (obj.transform.position.z < obj.wallMinZ)
        {
            obj.transform.position = new Vector3(obj.transform.position.x, obj.transform.position.y, obj.wallMinZ);
            obj.velocity.z = -obj.velocity.z * obj.restitution;
        }
        else if (obj.transform.position.z > obj.wallMaxZ)
        {
             obj.transform.position = new Vector3(obj.transform.position.x, obj.transform.position.y, obj.wallMaxZ);
            obj.velocity.z = -obj.velocity.z * obj.restitution;
        }
    }
}




[System.Serializable]
public class SimulatedObject
{
    public Transform transform;
    public float mass = 1f;
    public Vector3 velocity = Vector3.zero;
    public float damping = 0.1f;   // Damping force to slow down objects

    [Header("Collision")]
    public float floorY = 0f;       // Y position of the floor
    public float restitution = 0.7f; // Bounciness after collision. 0 = no bounce, 1 = perfect bounce
    public float friction = 0.2f;    // Friction when colliding with floor.
    public float wallMinX = -10f;
    public float wallMaxX = 10f;
    public float wallMinZ = -10f;
    public float wallMaxZ = 10f;

    [Header("External Forces")]
    public Vector3 additionalForce = Vector3.zero; // Apply an external force (e.g., wind)

    [Header("Physics Control")]
    public bool usePhysics = true;
}
```

**How to Use:**

1.  **Create a new Unity project (if you don't have one already).**
2.  **Create a new C# script** in your project named `PhysicsSimulator`.
3.  **Copy and paste the code** into the script.
4.  **Create a new C# script** named `SimulatedObject` (or use the name I gave it in the code).  Copy and paste the `SimulatedObject` class definition into it. (The second class definition in the code above).  *Important: Make sure this SimulatedObject class is outside of the PhysicsSimulator class. They are separate classes*.
5.  **Create some GameObjects** in your scene that you want to simulate. Add a Rigidbody component to them (even though we'll bypass the Unity physics).  These GameObjects will be controlled by the physics simulator.  *Important:* Remove the `useGravity` flag from the Rigidbody.
6.  **Create an empty GameObject** in your scene.  This will be your Physics Simulator manager.
7.  **Attach the `PhysicsSimulator` script** to the Physics Simulator manager object.
8.  **In the Inspector** for the Physics Simulator manager, you'll see the public variables.  Drag and drop your simulated GameObjects into the `Simulated Objects` list.
9.  **Adjust the `gravity`, `integrationSteps`**, and other parameters to your liking.
10. **Configure each SimulatedObject** by setting its mass, damping, collision parameters (floorY, restitution, friction), etc.

**Explanation:**

*   **`PhysicsSimulator` Class:** This is the main class that handles the simulation.
    *   `gravity`:  The gravitational acceleration.
    *   `integrationSteps`:  The number of integration steps per `FixedUpdate`. Higher values give more accuracy but are more computationally expensive.
    *   `simulatedObjects`:  A list of `SimulatedObject` instances that will be simulated.
    *   `FixedUpdate()`: The Unity callback function that is called at a fixed time interval, used for physics simulations. The simulation occurs here.
    *   `Simulate()`: This is the core of the simulation.  It iterates through the integration steps and applies the physics calculations to each `SimulatedObject`.
    *   `CalculateForces()`: Calculates the net force acting on the object (currently only gravity and any additional force you apply).
    *   `HandleCollisions()`:  A very basic collision detection and response function for the floor and walls.

*   **`SimulatedObject` Class:** This class holds the data for each simulated object.
    *   `transform`: A reference to the GameObject's transform.
    *   `mass`: The mass of the object.
    *   `velocity`: The object's velocity.
    *   `damping`:  A damping factor to simulate air resistance or friction.
    *   `floorY`:  The Y coordinate of the floor.
    *   `restitution`:  The coefficient of restitution (bounciness).
    *   `friction`:  Coefficient of friction with the floor.
    *   `additionalForce`: A force vector that can be used to apply constant forces (e.g., wind).
    *   `usePhysics`: A boolean flag to enable/disable physics for this specific object.

**Key Concepts:**

*   **Euler Integration:** A simple numerical integration method used to update the object's position and velocity. It's not the most accurate, but it's easy to implement. For more accurate results, consider using more advanced integration methods like Verlet integration or Runge-Kutta.
*   **FixedUpdate:**  Using `FixedUpdate` is crucial for physics simulations because it runs at a fixed time step, which is essential for deterministic and predictable behavior.
*   **Numerical Stability:** Increasing the `integrationSteps` can improve the stability of the simulation, especially for objects with high velocities or strong forces.

**Improvements:**

*   **More Advanced Collision Detection:**  Implement more robust collision detection algorithms (e.g., using raycasts, sphere casts, or bounding volumes). This example only includes basic floor and wall collisions.
*   **More Accurate Integration:**  Use Verlet integration or Runge-Kutta integration for improved accuracy and stability.
*   **Rotation:** Add support for simulating the rotation of objects.
*   **Constraints:** Implement constraints such as joints or springs.
*   **Optimization:**  Optimize the code for performance, especially if you have a large number of simulated objects.
*   **Custom Forces:** Add support for applying custom forces based on user input or other game logic.
*   **Avoid Unity Physics Interference:**  Disable Unity's built-in physics engine for the simulated objects to prevent conflicts.  The above implementation assumes you have removed `useGravity` from the Rigidbody.  For more complex scenarios, you might need to disable the entire physics engine.
*   **Error Handling:**  Add null checks and error handling to prevent crashes.
*   **Variable Time Step Handling:** Adapt the code to handle variable time steps gracefully if needed (though it's generally best to stick with a fixed time step for physics).

This is a basic framework.  Expand upon it to create a more complex and realistic physics simulator.  Remember to test thoroughly and optimize for your specific use case.