⚙️ Physics in Unity 3D

A comprehensive guide to the physics system in Unity 6: components, mechanisms, and best practices for creating realistic simulations.

📋 Table of Contents

Introduction to the Physics System
Main Components
Types of Colliders
Physics Materials
Joints & Constraints
Forces and Movement
Collision Detection
Optimization
Practical Examples

🎮 Introduction to the Unity Physics System

Unity's physics system is a simulation engine based on PhysX, which calculates realistic interactions between objects in the 3D world. Without adding code, an object with a Rigidbody will be attracted by gravity and react to collisions.

💡 Key Fact: The PhysX engine is used by the professional video game industry. It is optimized for CPU/GPU and produces predictable, consistent results.

Why does physics matter?

Realism: Objects move in a believable and predictable way
Gameplay: Game mechanics depend on physics (jumps, collisions, gravity)
Optimization: PhysX is highly optimized for production
Control: You can adjust every aspect for your specific game

⚠️ Technical Note: Physics in Unity is calculated at fixed intervals (Fixed Timestep) typically of 0.02 seconds. This is independent of the render FPS.

🔧 Main Components of the Physics System

For physics to work in Unity you need at least two components working together:

📦 Rigidbody

Function: Controls the movement and physics of a GameObject.

Requires: A Collider to interact.

Key properties:

Mass
Drag (Air resistance)
Angular Drag (Rotational resistance)
Gravity (Affected by gravity)
Constraints

⚡ Collider

Function: Defines the shape of the object to detect collisions.

Types: Box, Sphere, Capsule, Mesh, Wheel, Terrain.

Key properties:

Is Trigger
Material (Physics Material)
Offset
Size

Rigidbody States

State	Description	Use Cases
Dynamic	Full physics simulation. Affected by forces and collisions.	Moving objects (balls, boxes, enemies)
Kinematic	Moved by scripts, but dynamic objects bounce off it.	Moving platforms, doors
Static	Does not move. More efficient than dynamic.	Floors, walls, fixed obstacles

📐 Types of Colliders in Unity 3D

Each type of collider is optimized for different shapes and use cases:

Primitive

📦 Box Collider

Shape: Cube/Rectangle.

Use: Boxes, walls, platforms.

Advantage: Very fast, ideal for simple geometric objects.

CPU: ⭐⭐⭐ | Precision: ⭐⭐⭐

Primitive

🟢 Sphere Collider

Shape: Sphere.

Use: Balls, explosions, projectiles.

Advantage: Perfect for distance calculations and spherical detection.

CPU: ⭐⭐⭐ | Precision: ⭐⭐

Primitive

🔷 Capsule Collider

Shape: Cylinder with hemispheres.

Use: Characters, humanoid enemies.

Advantage: Excellent for movement without getting stuck in corners.

CPU: ⭐⭐⭐ | Precision: ⭐⭐⭐

Advanced

🧩 Mesh Collider

Shape: Exact geometry of the 3D model.

Use: Complex terrains, scenarios.

Warning: More expensive. Use sparingly.

CPU: ⭐ | Precision: ⭐⭐⭐⭐⭐

Specialized

🎡 Wheel Collider

Shape: Specialized wheel.

Use: Vehicles, cars, motorcycles.

Advantage: Realistic simulation of suspension and grip.

CPU: ⭐⭐ | Precision: ⭐⭐⭐⭐

Terrain

🗻 Terrain Collider

Shape: Height map.

Use: Wide terrains, mountains.

Advantage: Optimized for large areas.

CPU: ⭐⭐⭐ | Precision: ⭐⭐⭐

Compound Colliders

For complex objects, you can combine multiple primitive colliders on child objects:

GameObject Player
├─ Capsule Collider (body)
├─ Box Collider (arms)
└─ Sphere Collider (head)

Advantage: Greater precision than Mesh Collider, better performance than a single Mesh.

🎨 Physics Materials: Friction and Bounce

Physics Materials define how surfaces behave when they come into contact with other objects.

Main Properties

Dynamic Friction: Friction while sliding (0 = ice, 1 = sticky)
Static Friction: Friction at rest
Bounciness: Bounce (0 = no bounce, 1 = maximum bounce)
Friction Combine: How frictions are combined
Bounce Combine: How bounces are combined

Example Values

Material	Friction	Bounce
Ice	0.05	0.0
Asphalt	0.7	0.2
Tennis Ball	0.5	0.8
Rubber	1.0	0.3
Steel	0.4	0.9

Combination Modes

When two materials collide, they are combined using one of these modes:

Average

Average of both values. The most realistic.

Multiply

Multiplication. Results in lower values.

Minimum

Uses the smaller value (stickier/less bounce).

Maximum

Uses the larger value (slipperier/more bounce).

🔗 Joints: Constraints and Articulations

Joints connect two Rigidbodies and create constrained movement behaviours. They are essential for ragdolls, chains, and mechanisms.

📌 Fixed Joint ▼

Behaviour: Rigidly fixes two Rigidbodies in their relative position.

Use: Joined objects, connected machine parts.

Important parameter: Breaking Force (force needed to break the joint).

// Fixed Joint in code
Rigidbody rb1 = obj1.GetComponent<Rigidbody>();
Rigidbody rb2 = obj2.GetComponent<Rigidbody>();
FixedJoint joint = obj1.AddComponent<FixedJoint>();
joint.connectedBody = rb2;
joint.breakForce = 1000f;

🎪 Spring Joint ▼

Behaviour: Acts like an elastic spring between two Rigidbodies.

Use: Ropes, cables, vehicle suspensions.

Parameters:

Spring: Spring resistance (higher = stronger)
Damper: Damping (reduces oscillations)
Min/Max Distance: Rest distance

🔄 Hinge Joint ▼

Behaviour: Allows rotation on one axis (like a door).

Use: Doors, mechanical arms, levers.

Configuration: Define the axis of rotation (X, Y, or Z) and rotation limits.

// Hinge Joint - Door
HingeJoint hinge = doorGameObject.AddComponent<HingeJoint>();
hinge.axis = Vector3.up; // Rotation on Y axis
hinge.limits = new JointLimits() {
    min = 0,
    max = 90,
    bounciness = 0
};

👤 Character Joint ▼

Behaviour: Extended ball-and-socket joint for ragdolls.

Use: Ragdoll skeletons, human joints (arms, legs, neck).

Advantage: Allows complex rotation on multiple axes.

Parameters: Swing Limits (frontal rotation), Twist Limits (axial rotation).

⚙️ Configurable Joint ▼

Behaviour: The most flexible joint. Configurable in all degrees of freedom.

Use: Custom behaviours, specialized joints.

Complexity: High. Allows locking/unlocking 6 degrees of freedom (3 rotations + 3 translations).

Note: More computationally expensive than other joints.

🎯 Ball Joint ▼

Behaviour: Ball and socket like a hip joint.

Use: Spherical joints, hips, shoulders.

Feature: Allows rotation on all axes with specific limits.

💪 Applying Forces and Movement

There are multiple ways to move a Rigidbody. Each has its purpose:

⚡ AddForce()

Applies a continuous force that accumulates.

rigidbody.AddForce(Vector3.forward * 10);
// Gradually accelerates

Cases: Jumps, pushes, explosions.

🎯 Velocity

Sets the velocity directly.

rigidbody.velocity = Vector3.forward * 5;
// Instant velocity

Cases: Knockback, projectile movement.

🌪️ AddTorque()

Applies rotational force.

rigidbody.AddTorque(Vector3.right * 5);
// Gradual rotation

Cases: Spinning balls, rotating objects.

📍 MovePosition()

Interpolated kinematic movement.

rigidbody.MovePosition(transform.position + Vector3.forward);

Cases: Characters, moving platforms.

Force Modes

When using AddForce(), you can specify how the force is applied:

Force Mode	Formula	Use
Force (Default)	acceleration = Force / Mass	Real forces (wind, gravity, pushes)
Acceleration	Pure acceleration regardless of mass	When you want all objects to accelerate equally
Velocity	Direct velocity change	Smooth character movement
Impulse	Applied in one frame	Explosions, instant jumps

⚠️ Important: Always apply forces in FixedUpdate(), not in Update(). Physics runs in a separate Fixed Timestep.

void FixedUpdate() {
    rigidbody.AddForce(Vector3.forward * 10);
}

💥 Collision Detection

Unity offers three ways to detect collisions:

📞 OnCollisionEnter

Executes when two colliders start touching.

Requires: Both with Collider, at least one with Rigidbody.

void OnCollisionEnter(Collision collision) {
    Debug.Log("Collision!");
}

👀 OnTriggerEnter

Executes when an object enters a Trigger.

Requires: Collider with "Is Trigger" enabled.

void OnTriggerEnter(Collider collider) {
    Debug.Log("Entered!");
}

🎯 Physics.Raycast

Casts an invisible ray to detect collisions.

Use: Weapons, AI vision, terrain.

RaycastHit hit;
if (Physics.Raycast(pos, dir, out hit)) {
    Debug.Log(hit.collider.name);
}

Collision Detection Methods

The Rigidbody has options to detect collisions in different ways:

📊 Discrete (Default)

Detects collisions by checking position each frame. Faster, but can fail with very fast objects.

📏 Continuous

Calculates the full trajectory. Detects all collisions, slower.

⚡ Optimizing the Physics System

Physics can be expensive. Here are the best practices to optimize:

1️⃣ Use Primitive Colliders ▼

❌ Avoid: Mesh Colliders for dynamic objects.

✅ Do: Use Box, Sphere, Capsule Colliders.

Primitive colliders are 10-100x faster.

2️⃣ Mark static objects ▼

❌ Avoid: Dynamic Rigidbody for objects that don't move.

✅ Do: Leave as Static or use Kinematic only when necessary.

Static objects are not recalculated every frame.

3️⃣ Use Layers and Collision Matrix ▼

In Edit > Project Settings > Physics:

Define which layers can collide with each other
Example: Enemies don't need to collide with Enemies
Reduces unnecessary calculations

4️⃣ Compound Colliders ▼

For complex objects:

Use multiple primitive colliders on child objects
Better than a single Mesh Collider
Faster + better precision

5️⃣ Limit Dynamic Rigidbodies ▼

Each dynamic Rigidbody costs CPU:

Limit the number in scene (max ~100 at 60 FPS)
Use Kinematic for objects that don't need full physics
Destroy/Pool objects when possible

6️⃣ Adjust Fixed Timestep ▼

In Edit > Project Settings > Time:

Fixed Timestep: Default 0.02s (50 FPS)
Increase to 0.03s-0.05s for less precise games
Reduces FixedUpdate() calls

7️⃣ Use Constraints ▼

Freeze unnecessary axes:

2D character: Freeze Z and rotations
Object that doesn't rotate: Freeze all rotations
Reduces math calculations

💡 Pro Tip: Use Unity Profiler (Window > Analysis > Profiler) to measure physics cost. Look for "Physics" in the Profiler tab.

🎮 Practical Examples

Example 1: Character Movement

// Character with jump
public class PlayerController : MonoBehaviour {
    private Rigidbody rb;
    public float moveSpeed = 5f;
    public float jumpForce = 5f;

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

    void FixedUpdate() {
        // Horizontal movement
        float x = Input.GetAxis("Horizontal");
        rb.velocity = new Vector3(x * moveSpeed, rb.velocity.y, 0);

        // Jump
        if (Input.GetKeyDown(KeyCode.Space)) {
            rb.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
        }
    }
}

Example 2: Realistic Car Physics

// Suspension and traction
public class CarPhysics : MonoBehaviour {
    private Rigidbody rb;
    public WheelCollider[] wheels;
    public float motorForce = 3000;
    public float brakeTorque = 3000;

    void FixedUpdate() {
        float accel = Input.GetAxis("Vertical") * motorForce;
        float brake = Input.GetKey(KeyCode.Space) ? brakeTorque : 0;

        foreach (WheelCollider wheel in wheels) {
            wheel.motorTorque = accel;
            wheel.brakeTorque = brake;
        }
    }
}

Example 3: Ragdoll

// Enable/Disable ragdoll
void EnableRagdoll() {
    // Disable animator
    animator.enabled = false;

    // Enable all Rigidbodies
    Rigidbody[] allBodies = GetComponentsInChildren<Rigidbody>();
    foreach (Rigidbody body in allBodies) {
        body.isKinematic = false;
    }

    // The skeleton now simulates physics
}

Example 4: Projectile with Physics

// Realistic bullet/arrow
public class Projectile : MonoBehaviour {
    private Rigidbody rb;

    public void Launch(Vector3 direction, float force) {
        rb = GetComponent<Rigidbody>();
        rb.velocity = direction * force;

        // Rotate according to movement direction
        transform.rotation = Quaternion.LookRotation(direction);
    }

    void OnTriggerEnter(Collider collision) {
        if (collision.CompareTag("Enemy")) {
            Destroy(gameObject);
        }
    }
}

📚 Tabla de Referencia Rápida

Component/Method	Description	Performance
Rigidbody	Controls physics of an object	⭐⭐⭐
Box Collider	Cubic collider	⭐⭐⭐⭐⭐
Sphere Collider	Spherical collider	⭐⭐⭐⭐⭐
Capsule Collider	Cylindrical collider	⭐⭐⭐⭐⭐
Mesh Collider	Exact geometry collider	⭐⭐ (Avoid if possible)
Physics.Raycast	Detects collisions with a ray	⭐⭐⭐⭐
AddForce	Applies cumulative force	⭐⭐⭐⭐⭐
Velocity	Sets direct velocity	⭐⭐⭐⭐⭐
FixedJoint	Fixes two bodies together	⭐⭐⭐
SpringJoint	Creates elasticity	⭐⭐
HingeJoint	Rotation articulation	⭐⭐⭐
CharacterJoint	Complex articulation	⭐⭐

✅ Best Practices

✅ DO

Use primitive Colliders
Apply forces in FixedUpdate()
Use Raycasts for detection
Limit dynamic objects
Use Constraints to optimize
Profile performance regularly
Document physics values

❌ AVOID

Mesh Colliders on dynamic objects
Changing transform directly
Applying forces in Update()
Too many complex joints
Continuous Collision for everything
Ignoring the collision matrix
Physics without purpose

🔧 Troubleshooting

Q: My object falls through the floor ▼

A: Common issues:

The floor has NO Collider
The Rigidbody has "Is Kinematic" enabled
The object's velocity is too high (use Continuous Collision)
The Collider is too small (check "Edit Collider" in Inspector)

Q: My character doesn't move smoothly ▼

A: Solutions:

Apply forces in FixedUpdate(), not Update()
Use MovePosition() for smooth movement
Check Fixed Timestep (Project Settings > Time)
Make sure the Capsule Collider has the correct height

Q: Objects vibrate or behave erratically ▼

A: This is "jitter":

Increase Solver Iterations in Physics Settings
Reduce Force Scale and Penetration per Iteration
Make sure Colliders don't overlap
Check that mass is realistic (not too small)

Q: Joint breaks or strange behaviour ▼

A: Joint issues:

Verify that both Rigidbodies exist and are configured
Make sure the Connected Body is not null
Increase Solver Iterations if unstable
For ragdolls: configure Character Joints correctly

🎓 Conclusion

Unity's physics system is powerful but requires understanding. Remember:

Rigidbody + Collider = Base system (neither works without the other)
FixedUpdate() = The only correct way to apply forces
Primitives = Faster than Mesh Colliders
Physics Materials = Full control of behaviour
Joints = Create complex behaviours
Optimization = Essential for production games

The key is understanding the difference between what physics does automatically vs what needs to be controlled by code.

🚀 Next Steps:

Experiment with different values in Physics Settings
Create a small prototype: A ball rolling down a ramp
Build a simple ragdoll to understand Character Joints
Profile your game to identify physics bottlenecks

en-unity-physics-3d-guide

⚙️ Physics in Unity 3D

📋 Table of Contents

🎮 Introduction to the Unity Physics System

Why does physics matter?

🔧 Main Components of the Physics System

Rigidbody States

📐 Types of Colliders in Unity 3D

Compound Colliders

🎨 Physics Materials: Friction and Bounce

Main Properties

Example Values

Combination Modes

Average

Multiply

Minimum

Maximum

🔗 Joints: Constraints and Articulations

💪 Applying Forces and Movement

⚡ AddForce()

🎯 Velocity

🌪️ AddTorque()

📍 MovePosition()

Force Modes

💥 Collision Detection

Collision Detection Methods

⚡ Optimizing the Physics System

🎮 Practical Examples

Example 1: Character Movement

Example 2: Realistic Car Physics

Example 3: Ragdoll

Example 4: Projectile with Physics

📚 Tabla de Referencia Rápida

✅ Best Practices

✅ DO

❌ AVOID

🔧 Troubleshooting

🎓 Conclusion

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