HealthBar_KL.cs

    public Image healthBar;
    public float maximumHealth = 100f;
    public float currentHealth = 0f;


    void Start() {
        currentHealth = maximumHealth;
    }

    void Update() {

        if (currentHealth < 30.0f) {
            gameObject.GetComponent<Renderer>().material.color = Color.red;
        }
        if (currentHealth < 0.00f) {
            GameObject.Destroy(gameObject);
        }

    }

    public void onHit() {

        currentHealth -= 10f; // let's say the damage will be -10 per collision
        float calculate = currentHealth / maximumHealth;
        onSet(calculate);
    }

    void onSet(float hp) {
        healthBar.fillAmount = hp;
    }

    void OnCollisionEnter(Collision target) {

        // create a gameobject (a cube for this demo) name it "enemy"
        // and your argument(target) is setup so that object can be collided with
        if (target.gameObject.name == "enemy") {   
             
            onHit();
        }
    }