Why is the velocity of my rigid body in OnCollisionEnter always zero?

Question

My object must explode when the player touches it or when it collides at a low speed.

The problem I'm having is that its speed becomes zero when the floor collides for example and then the OnCollisionEnter event is called, it is already late and never fulfills the condition.

Someone knows what I'm doing wrong or what I should do to fix this?

void OnCollisionEnter(Collision other)
{
    Debug.Log(Mathf.Abs(rb.velocity.magnitude));
    if (Mathf.Abs(rb.velocity.magnitude) >= detonationImpactVelocity)
        Explosion(200);

    if (other.gameObject.tag == "Player")
        Explosion (600);

}

I came up with this but I guess there must be a better way to do it.

void Update()
{
    if (Mathf.Abs(rb.velocity.magnitude) >= detonationImpactVelocity)
        explosionSpeedAchieved = true;
    else
        explosionSpeedAchieved = false;
    Debug.Log(Mathf.Abs(rb.velocity.magnitude));
}

void OnCollisionEnter(Collision other)
{
    Debug.Log(Mathf.Abs(rb.velocity.magnitude));
    if (explosionSpeedAchieved)
        Explosion(200);

    if (other.gameObject.tag == "Player")
        Explosion (600);

}

Answer 1

Check instead the collision's relative velocity:

void OnCollisionEnter(Collision collision)
{
    Debug.Log(collision.relativeVelocity.magnitude);
    if (collision.relativeVelocity.magnitude >= detonationImpactVelocity)
        Explosion(200);

    if (collision.gameObject.CompareTag("Player"))
        Explosion (600);
}

On a side note, it would be more efficient to check the sqrMagnitude against the square of detonationImpactVelocity to avoid an unnecessary sqrt calculation.

Answer 2

You could try the relative linear velocity from the Collision object itself:

if (Mathf.Abs(other.relativeVelocity.magnitude) >= detonationImpactVelocity)

See also: https://docs.unity3d.com/ScriptReference/Collision-relativeVelocity.html