Rigidbody Physics: Collisions and Triggers

Unity's physics engine moves and collides GameObjects for you, once they have a Rigidbody (3D) or Rigidbody2D (2D) component and a Collider.

using UnityEngine;

public class Bullet : MonoBehaviour
{
    public float speed = 20f;

    void Start()
    {
        GetComponent<Rigidbody>().linearVelocity = transform.forward * speed;
    }

    void OnCollisionEnter(Collision collision)
    {
        Debug.Log($"Hit {collision.gameObject.name}");
        Destroy(gameObject);
    }
}

Collisions vs triggers

• A regular collision (OnCollisionEnter/Stay/Exit) needs both objects to have solid (non-trigger) colliders, physics stops them from overlapping.
• A trigger collider (Is Trigger checked) lets objects pass through it, but still fires OnTriggerEnter/Stay/Exit, useful for pickups, checkpoints, or damage zones that shouldn't physically block anything.
• Both events only fire if at least one of the two objects also has a Rigidbody.

void OnTriggerEnter(Collider other)
{
    if (other.CompareTag("Player"))
    {
        Debug.Log("Player entered the zone");
    }
}

Physics update, not Update

Move a Rigidbody with Rigidbody.linearVelocity or AddForce inside FixedUpdate(), not Update(), FixedUpdate runs on physics's fixed timestep, keeping movement consistent regardless of framerate.