void OnCollisionEnter(
    Collision collision)
{
    if (collision.gameObject.CompareTag("Enemy"))
    {
        TakeDamage(10);
    }
}

Impact damage:

float impact =
    collision.relativeVelocity.magnitude;

if (impact > 10f)
{
    TakeDamage(
        Mathf.RoundToInt(impact)
    );
}