Unity : Physics and Collisions

Physics and Collisions in Unity

Unity’s Physics engine allows developers to simulate real-world interactions like movement, gravity, and collisions. Collisions in Unity occur when Colliders from two GameObjects come into contact. Depending on the setup, you can create realistic effects such as bouncing, sliding, or stopping.

Physics is essential for creating dynamic gameplay, whether it’s a falling cube, a rolling ball, or a character jumping in a platformer.

Rigidbody and Collider Components

To make a GameObject interact with Unity’s physics system, you typically use two key components:

• Rigidbody: Adds physical properties like mass, velocity, drag, and gravity. Without a Rigidbody, objects will not respond to forces.
• Collider: Defines the shape for collision detection (Box, Sphere, Capsule, or Mesh Colliders). Colliders can be set to solid or trigger mode.

For example, a rolling ball would have a Sphere Collider and a Rigidbody to make it move and react to the environment.

Detecting Collisions and Triggers

Unity provides two main ways to detect interactions between objects:

1. Collisions: Occur when two Colliders touch and at least one has a Rigidbody. Use OnCollisionEnter, OnCollisionStay, and OnCollisionExit in a script to respond to these events.
2. Triggers: Colliders with Is Trigger enabled that allow objects to pass through while still detecting overlaps. Handle these with OnTriggerEnter, OnTriggerStay, and OnTriggerExit.

Example: Collision Detection

// Attach this script to a GameObject with a Collider and Rigidbody
void OnCollisionEnter(Collision collision)
{
    Debug.Log("Collided with: " + collision.gameObject.name);
}
      

Example: Trigger Detection

// Attach this script to a GameObject with a Collider (Is Trigger enabled)
void OnTriggerEnter(Collider other)
{
    Debug.Log("Entered trigger with: " + other.gameObject.name);
}
      

Triggers are useful for areas like checkpoints, item pickups, or hazard zones.

Physics Materials and Gravity

Unity uses Physics Materials to control how surfaces interact during collisions:

• Bounciness: How much an object rebounds after impact.
• Friction: How easily objects slide or stop when in contact.

By adjusting physics materials, you can create ice-like slippery surfaces or rubbery, bouncy behaviors. Additionally, Rigidbody settings allow you to enable or disable gravity, which is useful for floating objects or space-based mechanics.

Simple Physics-Based Gameplay

Physics can bring your gameplay to life with minimal code. Here are some examples of simple physics-based mechanics:

• Rolling balls down ramps using gravity and friction.
• Making crates fall and stack in a puzzle game.
• Launching objects using Rigidbody.AddForce() for explosions or jumps.
• Creating triggers for collectibles, checkpoints, or trap zones.

Example: Applying Force to a Rigidbody

public class BallController : MonoBehaviour
{
    public Rigidbody rb;
    public float force = 500f;

    void Start()
    {
        rb = GetComponent();
    }

    void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            // Add an upward force to simulate a jump
            rb.AddForce(Vector3.up * force);
        }
    }
}
      

Combining Rigidbody dynamics, Colliders, Triggers, and Physics Materials lets you create immersive and interactive environments that are the foundation of many Unity games.