Unity Term Book
Physics

Rigidbody

Rigidbody hands a GameObject to the physics engine — gravity, forces, velocity, and collisions are computed by Unity Physics instead of your Update loop.

Imagine...

A Rigidbody is like an object's physics soul. Before it exists, a box is just a still image hanging in the air. After you add a Rigidbody, it becomes a body with real mass — pulled down by gravity, bouncing on impact, sliding under force. You push it with `AddForce()`, like a hand, instead of writing `transform.position` directly.

The concept in detail

A Rigidbody registers the GameObject with Unity Physics. Main properties: mass (kg), drag (air resistance), angularDrag (rotational resistance), useGravity, isKinematic.

ForceMode: Force (continuous, uses mass), Acceleration (continuous, ignores mass), Impulse (instant, uses mass — typical for jump), VelocityChange (instant, ignores mass).

Kinematic Rigidbody: isKinematic = true — still participates in collision detection but is not driven by forces or gravity. Move it with MovePosition(). Use this for moving platforms and animated objects.

Critical rule: never set transform.position directly on a non-kinematic Rigidbody — that teleports the body and breaks the simulation. Always use AddForce() or velocity.

Diagram: Forces & ForceMode

ForceMode Types

Force

Continuous, uses mass. Rockets, wind...

rb.AddForce(dir * 10f)

Impulse

Instant, uses mass. Jumps, explosion knockback

rb.AddForce(Vector3.up * 5f, ForceMode.Impulse)

VelocityChange

Instant, ignores mass. Sets velocity directly

rb.AddForce(dir, ForceMode.VelocityChange)

Rigidbody Properties

mass1.0 kg
drag0.0
angularDrag0.05
useGravitytrue
isKinematicfalse

Constraints (Freeze)

☑ Freeze Pos X☑ Freeze Rot Z□ Freeze Pos Y□ Freeze Rot X

Hands-on steps

1

Add a Rigidbody component

Inspector → Add Component → Physics → Rigidbody. Also add a Collider if you want collisions.

2

Cache the Rigidbody in Awake

_rb = GetComponent<Rigidbody>() — then use it in FixedUpdate.

3

Move with forces in FixedUpdate

Do not write transform.position. Use AddForce() or set velocity.

4

Freeze Rotation for 2D top-down

Inspector → Constraints → Freeze Rotation X, Y, Z so the object does not tumble on impact.

5

Use isKinematic for moving platforms

A platform driven by animation → isKinematic = true + MovePosition().

Interactive simulator

Click the canvas to spawn objects. Tweak Gravity, Mass, and Drag, then press Force Up to apply an impulse.

Objects: 0|Gravity: 9.8 m/s²|Click the canvas to spawn a Rigidbody

Code example

Basic

Move and jump with a Rigidbody.

using UnityEngine;

public class PhysicsPlayer : MonoBehaviour
{
  [SerializeField] private float moveForce = 10f;
  [SerializeField] private float jumpForce = 7f;
  [SerializeField] private float maxSpeed  = 8f;
  private Rigidbody _rb;
  private Vector3   _input;
  private bool      _grounded;

  void Awake() => _rb = GetComponent<Rigidbody>();

  void Update()
  {
      _input = new Vector3(Input.GetAxisRaw("Horizontal"), 0,
                           Input.GetAxisRaw("Vertical"));
      if (Input.GetButtonDown("Jump") && _grounded)
          _rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
  }

  void FixedUpdate()
  {
      // Cap speed on the horizontal plane
      Vector3 flatVel = new Vector3(_rb.velocity.x, 0f, _rb.velocity.z);
      if (flatVel.magnitude > maxSpeed)
          _rb.velocity = flatVel.normalized * maxSpeed + Vector3.up * _rb.velocity.y;

      _rb.AddForce(_input.normalized * moveForce, ForceMode.Force);
  }

  private void OnCollisionEnter(Collision col)
      => _grounded = col.gameObject.CompareTag("Ground");

  private void OnCollisionExit(Collision col)
      => _grounded = false;
}

Code example

Advanced

Explosion force with AddExplosionForce — fling bodies away from the blast center.

using UnityEngine;

public class Explosion : MonoBehaviour
{
  [SerializeField] private float force     = 500f;
  [SerializeField] private float radius    = 5f;
  [SerializeField] private float upModifier = 0.5f;

  public void Explode()
  {
      // Find every Collider inside the radius
      Collider[] cols = Physics.OverlapSphere(transform.position, radius);

      foreach (var col in cols)
      {
          var rb = col.GetComponent<Rigidbody>();
          if (rb == null) continue;

          // Unity scales the force by distance
          rb.AddExplosionForce(force, transform.position, radius, upModifier);
      }

      // Visual effect + destroy
      GetComponent<ParticleSystem>()?.Play();
      Destroy(gameObject, 2f);
  }
}

📌 Quick recap

  • ▸Forces → FixedUpdate; Input → Update
  • ▸Jump: AddForce(up, ForceMode.Impulse)
  • ▸Never set transform.position on a non-kinematic RB
  • ▸Moving platform → isKinematic + MovePosition

⚠️ Common mistakes

  • ❌ Writing transform.position on a Rigidbody

    Teleport — breaks physics, wall clipping, collision glitches

    ✅ rb.MovePosition() or velocity

  • ❌ Physics code in Update instead of FixedUpdate

    Out of sync with the physics timestep → jitter, tunneling

    ✅ Every AddForce/velocity call → FixedUpdate