【Unity】Unity's Physics Core: Complete Guide to Rigidbody

Overview

One of Unity's powerful features is its built-in physics engine (NVIDIA PhysX) for easy realistic physics simulation. Objects falling due to gravity, accelerating when force is applied, bouncing off each other on collision—all these real-world behaviors are calculated by this physics engine.

Rigidbody is the component that places GameObjects under physics engine control, making them behave as "physics-enabled objects." Unlike directly manipulating the Transform component, using Rigidbody enables more realistic and dynamic movement.

This article covers the basic role of the Rigidbody component and proper methods for moving objects using Rigidbody.

The Role of Rigidbody

Adding a Rigidbody component to a GameObject makes it a "mass-bearing object" subject to physical effects:

Gravity: With Use Gravity enabled, objects automatically fall toward gravity (default: negative Y-axis direction).
Forces and Torque: Apply AddForce from scripts to accelerate, AddTorque to rotate.
Collision: Collides with other Rigidbody or Collider objects, producing realistic rebounds and pushes based on mass and velocity.
Drag: Set Drag (air resistance) and Angular Drag (rotational resistance) to express movement damping.

`transform` vs `Rigidbody`: Moving Objects

The most confusing point for beginners is the difference between moving with transform.Translate() versus Rigidbody.

transform.Translate(): Essentially "teleports" the object by the specified amount. Completely ignores physics laws, potentially causing objects to clip through walls or pass through other objects. Suitable for simple movement without physics interactions, or objects outside physics engine control (e.g., camera movement).
Rigidbody operations: Moves objects by applying force or setting velocity directly. All movement is calculated and managed by the physics engine, producing realistic collisions and interactions with other objects. Objects requiring physics behavior should always be moved through Rigidbody.

Warning: Directly manipulating the transform of objects with Rigidbody causes conflicts with physics engine calculations, resulting in teleportation and unnatural collisions. Avoid mixing both approaches.

Moving Objects with Rigidbody

To operate a Rigidbody, first get its reference with GetComponent<Rigidbody>() and process in FixedUpdate().

1. Applying Force with `AddForce()`

AddForce() is the most common method for continuously applying force to accelerate objects. It's analogous to "pushing" in the real world.

using UnityEngine;

[RequireComponent(typeof(Rigidbody))]
public class PlayerController : MonoBehaviour
{
    public float moveForce = 10f;
    private Rigidbody rb;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    void FixedUpdate()
    {
        float moveHorizontal = Input.GetAxis("Horizontal");
        float moveVertical = Input.GetAxis("Vertical");

        Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

        // Apply force to Rigidbody
        rb.AddForce(movement * moveForce);
    }
}

AddForce has an optional ForceMode argument specifying how force is applied:

ForceMode.Force: Continuous force considering mass (default).
ForceMode.Acceleration: Continuous force ignoring mass.
ForceMode.Impulse: Instantaneous impulse considering mass (for explosions, etc.).
ForceMode.VelocityChange: Instantaneous velocity change ignoring mass.

2. Setting Velocity Directly

Directly modifying rb.velocity sets the object's velocity immediately. Useful for snappier character control.

void FixedUpdate()
{
    float moveHorizontal = Input.GetAxis("Horizontal");
    float moveVertical = Input.GetAxis("Vertical");

    Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

    // Preserve Y velocity while setting X, Z velocity
    rb.velocity = new Vector3(movement.x * speed, rb.velocity.y, movement.z * speed);
}

Key Properties

Mass: Object mass. Heavier objects are harder to move and have greater impact on other objects.
Drag: Air resistance during movement. Higher values cause faster velocity decay.
Angular Drag: Rotational resistance. Higher values stop rotation faster.
Use Gravity: Whether to be affected by gravity.
Is Kinematic: When enabled, the object ignores physics engine forces (gravity, collisions). However, moving via transform can still physically affect other Rigidbody objects (non-kinematic). Used for animation-driven characters or script-controlled platforms.
Constraints: Freeze movement or rotation on specific axes. Useful for preventing 2D characters from moving on Z-axis, or stopping player characters from toppling by freezing X, Z rotation.

Summary

Rigidbody is the protagonist of Unity's physics world.

Adding Rigidbody makes objects obey physics laws.
Move physics-enabled objects through Rigidbody, not transform.
Use AddForce() for continuous force, velocity for direct speed control.
Write processing in FixedUpdate()—this is essential.
Is Kinematic is a special mode for affecting others without receiving physics.

Mastering Rigidbody is the key to creating realistic, compelling interactions.

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