Can Someone Please Tell Me Why This Doesn't Work!

Questions & Answers
1

I’m trying to make a collision/health system, however nothing is being added to the hitAmount value even when the applied force is more than 3.5.

var hitAmount : float;
var appliedForce : float;

function Update () {
    if(appliedForce >= 3.5)
        hitAmount +=10.0;

    if(hitAmount >= 100.0)
        Destroy(gameObject);
}

function OnCollisionEnter () {
    appliedForce = rigidbody.velocity.magnitude / 10;
}
2

As @Eric5h5 and @DaveA said, you should not increment hitAmount in Update: if a collision sets appliedForce to a high enough value, hitAmount will be incremented each frame by 10, destroying your object in 10 frames or less. You should compare appliedForce in the collision event:

    var hitAmount : float;
    var appliedForce : float;

    function Update () {
      if(hitAmount >= 100.0)
        Destroy(gameObject);
    }

    function OnCollisionEnter () {
        appliedForce = rigidbody.velocity.magnitude / 10;
        if(appliedForce >= 3.5)
            hitAmount +=10.0;
    }

But there’s another important issue: you’re just checking if the rigidbody has an absolute velocity greater than 35m/s (too high!) when colliding, no matter which’s the other object’s velocity. You should check the relative velocity, like this:

function OnCollisionEnter(col: Collision){
  if (col.relativeVelocity.magnitude > someLimit){
    hitAmount += 10.0;
  }
}

As I said, 35m/s is a too high velocity: it’s 0.7m per physics cycle, what means that objects thinner than 0.7m may be just ignored by the collision system. You should use a smaller limit - maybe 5 or 10 m/s - as someLimit in the OnCollisionEnter event above.