ConstantForce not working when used in separate PhysicsScene

Unity Engine
1

We encountered a problem where a ConstantForce is not applied in a separate PhysicsScene, but is working in our realtime scene. We use a separate scene to calculate the path of a bullet to preview it to the user.

I stripped the problem down to a very simple script that anyone can try. This script executes 10 steps of a physics simulation in two versions: One version with constant force, the second version without.
The resulting position of the test object should not be identical, but it is.

Reproduction should be easy:

  • Create an empty scene
  • Create an empty game object
  • Attach the script below
  • Press play
  • Use the two check boxes in the inspector to run two versions of the simulation and check the debug log

Are we missing something or is this a Unity bug? We use 2020.3.15f2

using System.Collections;
using UnityEngine;
using UnityEngine.SceneManagement;

public class PhysicsSimulationTest : MonoBehaviour
{
    [SerializeField] private bool runWithConstantForce;
    [SerializeField] private bool runWithoutConstantForce;

    void RunTest(bool withConstantForce)
    {
        // create a separate scene to simulate physics
        var testScene = SceneManager.CreateScene("PhysicSimulation", new CreateSceneParameters(LocalPhysicsMode.Physics3D));

        // set test scene active
        var current = SceneManager.GetActiveScene();
        SceneManager.SetActiveScene(testScene);

        // create simple game object with a rigidbody and a mass of 1
        var bulletPhysic = new GameObject();

        var rb = bulletPhysic.AddComponent<Rigidbody>();
        rb.mass = 1;
        rb.position = Vector3.zero;

        if (withConstantForce)
        {
            // apply a constant force to move the object upwards
            var cf = bulletPhysic.AddComponent<ConstantForce>();
            cf.force = new Vector3(0, 1000, 0);
        }

        // move object sideways during simulation
        rb.AddForce(new Vector3(10,0,0), ForceMode.Impulse);

        // execute 10 steps and log the position after each step
        for (var i=0;i<10;++i)
        {
            var physicsScene = testScene.GetPhysicsScene();
            physicsScene.Simulate(Time.fixedDeltaTime);

            Debug.Log("Pos: " + rb.position);
        }

        // cleanup
        Destroy(bulletPhysic);

        SceneManager.SetActiveScene(current);
       
        StartCoroutine(Unload(testScene));
    }

    private static IEnumerator Unload(Scene testScene)
    {
        yield return SceneManager.UnloadSceneAsync(testScene);
    }

    private void OnValidate()
    {
        if (runWithConstantForce)
        {
            runWithConstantForce = false;
            RunTest(true);
        }
        else if (runWithoutConstantForce)
        {
            runWithoutConstantForce = false;
            RunTest(false);
        }
    }
}
2

ConstantForce is a physics component. It runs during the fixed-update and you’re not running that, you’re simply performing 10 executions of the simulation, not 10 executions of the FixedUpdate.

It’s not an inherent part of the Rigidbody. If you want a constant force then add a constant force to the Rigidbody.