Physics-Based Puzzles and Gameplay

# CHAPTER 12

Physics-Based Puzzles and Gameplay #

1. Introduction #

2. Learning Objectives #

• Design weight-sensitive pressure plates using Mass calculations.

• Construct interactive mechanisms (levers and doors) using Hinge Joints.

• Implement Push/Pull mechanics for heavy objects.

• Program an object-grabbing system (like the Gravity Gun).

• Prevent common physics exploits in puzzle design.

3. Weight-Based Triggers #

• We must extract the Rigidbody of the object entering the trigger and check its .mass property.

public float requiredMass = 50f;
private float currentMassOnPlate = 0f;

void OnTriggerEnter(Collider other)
{
    Rigidbody rb = other.GetComponent<Rigidbody>();
    if (rb != null)
    {
        currentMassOnPlate += rb.mass;
        if (currentMassOnPlate >= requiredMass)
        {
            OpenPuzzleDoor();
        }
    }
}

4. Mechanical Joints (Levers and Hinges) #

• Attach a Hinge Joint to the lever.

• Set the Axis of the hinge so it only rotates forward and backward.

• Set the Limits (e.g., -45 degrees to +45 degrees).

5. Detecting Joint Angles #

• You read the angle of the Hinge Joint in the Update loop.

public HingeJoint leverJoint;

void Update()
{
    // If the lever is pushed all the way to 45 degrees
    if (leverJoint.angle >= 45f)
    {
        Console.WriteLine("Lever Activated!");
    }
}

6. The "Gravity Gun" (Grabbing Objects) #

• Bad Way: Making the box a child of the player. This ruins the physics simulation, and the box will clip through walls.

• Good Way: Create a FixedJoint (or a very stiff SpringJoint) between the Player's hands and the Box. The box becomes magically tethered to the player, but if the player turns too fast and the box hits a wall, the physics engine calculates the collision properly and the box will drop!

7. Visual Learning: The Weight Plate Puzzle #

[ 10kg Crate ]   +   [ 40kg Player ]
      \                   /
       \                 /
   [ TRIGGER ZONE (Requires 50kg) ]
               |
         [ DOOR OPENS ]

8. Best Practices #

• Visual Feedback: A physics puzzle is frustrating if the player doesn't know it's working. If a player places a 10kg block on a 50kg plate, the plate should visually sink downwards *slightly* (perhaps 20% of the way) using C# Lerp, showing the player they are making progress.

9. Common Mistakes #

• The Stacking Exploit: If you require the player to reach a ledge 10 meters high, and you put 10 tiny crates in the room, the player will try to stack them. As mentioned in Chapter 9, stacking 10 rigidbodies causes massive instability. The tower will violently explode, frustrating the player. If you want a stacking puzzle, use Snap-To-Grid mechanics, or limit the required stack to 3 items.

10. Mini Project: Build a Pushable Object System #

class CharacterPusher : MonoBehaviour
{
    public float pushPower = 5.0f;

    // A built-in Unity event that fires when a CharacterController hits something
    void OnControllerColliderHit(ControllerColliderHit hit)
    {
        Rigidbody body = hit.collider.attachedRigidbody;

        // Ensure the object has a Rigidbody and isn't frozen
        if (body == null || body.isKinematic) return;

        // Calculate the push direction (ignore vertical pushing)
        Vector3 pushDir = new Vector3(hit.moveDirection.x, 0, hit.moveDirection.z);

        // Apply force to the box based on our speed
        body.velocity = pushDir * pushPower;
    }
}

11. Practice Exercises #

1. 1. Why is it important to check the rb.mass property inside a puzzle trigger rather than just relying on OnTriggerEnter?

1. 2. What type of Physics Joint should be used to create a realistic, physical trapdoor that swings downward when stepped on?

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the architecture of a weight-based pressure plate. How do you track the total mass dynamically when multiple objects enter and exit the trigger zone?

• Q: Contrast the implementation of an animated door versus a physics-driven door using a Hinge Joint. What are the emergent gameplay benefits of the physics-driven approach?

• Q: A developer complains that their Kinematic character simply clips into Rigidbody crates instead of pushing them. Explain why the physics engine ignores the character, and how to resolve this using scripted force application.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #