[Solved] 0g collisions and an obvious violation of Newton's third law of motion!!

I am working on a space simulator game and require realistic collisions of rigid bodies with various masses.

I have tried to build a scene where one free floating mass pushes against another free floating mass but the results were less than stunning.

The piston is simply a child of the of the larger object it is connected to, and both objects have box colliders and a mass of 1000 units.

When the two bodies impact they do appear to push away from eachother with eual force (which looks correct), however once the collision ends, the bodies appear to stop in place as if they have no added velocity.

Could someone please help me understand why unity is not applying the proper acceleration/velocity to these bodies?

 

Thanks for pointing that out, and sorry I had it set to private, but should be fixed now.

 

its a very simple repositioning script:

Code (csharp):

1.  
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class Piston : MonoBehaviour {
7.  
8.     [SerializeField]
9.     float rate = 1f;
10.  
11.     [SerializeField]
12.     Vector3 dir;
13.  
14.     Vector3 initipos;
15.  
16.     [SerializeField]
17.     Transform piston;
18.     // Use this for initialization
19.     void Start () {
20.         initipos = piston.localPosition;
21.     }
22.    
23.     // Update is called once per frame
24.     void Update () {
25.         piston.localPosition = initipos + Mathf.Sin ((Mathf.PI ) * Time.time * rate) * dir;
26.     }
27. }
28.  

 

Great example! Worth mentioning that Rigidbody.MovePosition is called from FixedUpdate to produce the effect correctly.

 

Thank you MelvMay for the hint about programmatically adding momentum, I was able to create a working concept that does look convincing for multiple free floating masses:

Code (csharp):

1. using System.Collections;
2. using UnityEngine;
3.  
4. public class ApplyRBVelocity : MonoBehaviour {
5.     /*simply attach this script to a free floating rigid body which is non kinematic and use gravity is false*/
6.     Rigidbody rb;
7.     Vector3 previous = Vector3.zero;
8.  
9.     void Awake() {
10.         rb = GetComponent<Rigidbody> ();
11.         previous = rb.position;
12.     }
13.     void OnCollisionEnter(Collision col) {
14.         previous = transform.position;
15.     }
16.     void OnCollisionStay(Collision col) {
17.         doOnFixedUpdate = doOnFixedUpdate != null ? doOnFixedUpdate : StartCoroutine (DoOnFixedUpdate());
18.     }
19.  
20.     Coroutine doOnFixedUpdate;
21.     IEnumerator DoOnFixedUpdate() {
22.         yield return new WaitForFixedUpdate ();
23.  
24.         var velocity = (1f / Time.fixedDeltaTime) * (transform.position - previous) ;
25.         previous = transform.position;
26.  
27.         rb.velocity = velocity;
28.         //Debug.DrawRay (rb.transform.TransformPoint(rb.centerOfMass), velocity * 10, Color.cyan);
29.         doOnFixedUpdate = null;
30.     }
31. }

Thanks.