unity-physics

Unity Physics

Physics System Overview

Unity provides different physics engine integrations for different project needs:

• 3D Physics (PhysX): Nvidia PhysX engine integration for 3D object-oriented projects
• 2D Physics (Box2D): Optimized 2D physics system with dedicated components
• Both systems simulate gravity, collisions, forces, and constraints

Physics Timing

Physics runs on a fixed timestep via FixedUpdate, separate from the rendering frame rate:

void FixedUpdate()
{
    // All physics code belongs here, not in Update()
    rb.AddForce(Vector3.forward * speed);
}

Key timing rules:

• FixedUpdate runs at a fixed interval (default 0.02s / 50Hz)
• Multiple FixedUpdate calls can occur per frame, or none
• Time.fixedDeltaTime controls the interval
• Use Update for input, FixedUpdate for physics forces

Simulation Modes

Physics.simulationMode controls when the physics engine steps:

• FixedUpdate (default): Automatic simulation each fixed timestep
• Update: Simulate once per frame
• Script: Manual control via Physics.Simulate()

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Rigidbody Configuration

A Rigidbody component places a GameObject under physics engine control. A rigid body does not deform or change shape under physics forces.

Key Properties

Property	Description	Default
mass	Mass in kg; affects force interactions	1
linearDamping	Resistance to linear velocity	0
angularDamping	Resistance to angular velocity	0.05
useGravity	Whether gravity affects this body	true
isKinematic	If true, not driven by physics forces	false
interpolation	Smooths visual jitter between physics steps	None
collisionDetectionMode	Algorithm for detecting collisions	Discrete

Movement Methods

Rigidbody rb = GetComponent<Rigidbody>();

// Apply continuous force (call in FixedUpdate)
rb.AddForce(Vector3.forward * 10f);
rb.AddForce(Vector3.up * 5f, ForceMode.Impulse);

// Apply torque
rb.AddTorque(Vector3.up * 2f);

// Apply force at a world position (creates both force and torque)
rb.AddForceAtPosition(Vector3.forward * 10f, hitPoint);

// Kinematic movement (use for isKinematic=true bodies)
rb.MovePosition(rb.position + direction * speed * Time.fixedDeltaTime);
rb.MoveRotation(targetRotation);