07: Rude Gold Bug (Part 5)

Step 1: Scripting Basics — Triggers and Collisions

So far your machine has used physics components set up entirely in the Inspector. In this part, you'll write C# scripts that react to physics events and change what objects do at runtime.

There are two main ways a script can detect physical contact:

• • Trigger Colliders — A collider with "Is Trigger" checked. Objects pass through it, but the script is notified. Good for invisible zones.
  • Collision Events — Two solid colliders that actually bounce off each other. The script is notified when they first touch.

The relevant Unity event methods are:

Example 1: Trigger Applies Force or Torque

When the ball (or any object) enters a trigger zone, the script can push or spin another object by calling AddForce() or AddTorque() on its Rigidbody.

Store the target object as a public variable so you can assign it in the Inspector without hard-coding names:

using UnityEngine;

public class TriggerForce : MonoBehaviour
{
    public Rigidbody target;          // drag the object here in Inspector
    public Vector3 force  = new Vector3(0, 500, 0);
    public Vector3 torque = new Vector3(0, 0, 200);

    void OnTriggerEnter(Collider other)
    {
        if (target != null)
        {
            target.AddForce(force,  ForceMode.Impulse);
            target.AddTorque(torque, ForceMode.Impulse);
        }
    }
}

Example 2: Trigger Releases a Kinematic Object

A kinematic Rigidbody is frozen in place — gravity and forces have no effect on it. Setting isKinematic = false at runtime "releases" it so it falls or flies away normally.

using UnityEngine;

public class ReleaseKinematic : MonoBehaviour
{
    public Rigidbody target;   // the frozen object to release

    void OnTriggerEnter(Collider other)
    {
        if (target != null)
        {
            target.isKinematic = false;  // let gravity take over
        }
    }
}

Example 3: Collision Between Two Objects Changes Movement

OnCollisionEnter fires when two non-trigger colliders physically hit each other. You can react to that impact — add extra force, change kinematic state, or anything else.

using UnityEngine;

public class CollisionBumper : MonoBehaviour
{
    public float bounceForce = 800f;

    void OnCollisionEnter(Collision col)
    {
        // Push the incoming object away from this object's center
        Rigidbody rb = col.rigidbody;
        if (rb != null)
        {
            Vector3 direction = col.transform.position - transform.position;
            direction.Normalize();
            rb.AddForce(direction * bounceForce, ForceMode.Impulse);
        }
    }
}

Example 4: Trigger Changes Other Object's State

Not every trigger needs to use forces or kinematic. You can change almost anything in your scene from a script — a light's color, a Hinge Joint's motor speed, a spring's strength, an object's material, whether an object is active at all.

using UnityEngine;

public class TriggerStateChange : MonoBehaviour
{
    public HingeJoint motorJoint;      // drag the hinge here
    public float targetMotorSpeed = 200f;

    void OnTriggerEnter(Collider other)
    {
        if (motorJoint != null)
        {
            JointMotor motor = motorJoint.motor;
            motor.targetVelocity = targetMotorSpeed;
            motor.force          = 100f;
            motorJoint.motor     = motor;
            motorJoint.useMotor  = true;
        }
    }
}

Step 2: Add Scripts to Your Scene

Now it's your turn!

Your scripts must include at least two of the four interaction types described above. Each script must actually affect the Rude Gold Bug's journey — it should contribute directly or indirectly to the ball's motion through the machine.

• Create your scripts in a Scripts subfolder inside your Assets folder
• Attach each script to the appropriate object in your scene
• Assign any public variables (like target) by dragging objects into the Inspector
• Use trigger colliders (Is Trigger = true) where your zone should not block movement
• Test carefully — make sure each script fires at the right moment

Step 3: Testing and Debugging

Scripts introduce new ways things can go wrong. Follow this process:

1. 1. Check the Console window for errors — a red error usually means something is null or misnamed
   2. Use Debug.Log("trigger fired!") inside your event method to confirm it is being called
   3. Check that Is Trigger is checked on your trigger collider (and that the other object has a Rigidbody)
   4. If AddForce seems to do nothing, make sure isKinematic is false on the target
   5. Iterate — adjust force values until the motion feels right