Calculate the distance for activate a rigidbody

Hi community, I'm currently working on my physics game and in my game, I need to change the character's movement speed but always keep a rigidbody falling in just front of it. For example:

Code (CSharp):

1. public class Test : MonoBehaviour
2. {
3.     public Transform cube1;
4.     public Transform cube2;
5.  
6.     public float activeDist = 4.0f;
7.     public float speed = 2.0f;
8.  
9.     private void Update()
10.     {
11.         cube1.Translate(Vector3.right * Time.deltaTime * speed);
12.         float currentDist = (cube2.position - cube1.position).sqrMagnitude;
13.  
14.         if (currentDist < activeDist * activeDist)
15.         {
16.             if (cube2.GetComponent<Rigidbody>().isKinematic)
17.             {
18.                 cube2.GetComponent<Rigidbody>().isKinematic = false;
19.             }
20.         }
21.     }
22. }

I'm not an expert in physics calculation. Let's say the current movement speed and the active distance works just fine. How can I calculate the activate distance for different movement speed but achieve the same result? And without changing the gravity. Thanks for any help.

 

Interesting problem
If I get you right, you want blue object fall just before red, if in the range.

I would approach it this way.
- Get vertical distance of blue object.
- Calculate duration of fall, based on gravity.
Knowing the duration
- Calculate velocity of red object
- Calculate distance to travel by red object, within fall duration of blue object.
- Calculate distance from red object to drop point of blue object (in 2D that is just single axis (right))
- Calculate difference between drop point distance and distance to travel withing fall duration.
- You may need add extra offset velocity, to the calculated difference, to ensure blue box falls ahead the red one.
Then
- If difference with offset is 0, drop the blue object.

For first two parts, check out this link with simple equations
https://en.wikipedia.org/wiki/Equations_for_a_falling_body