unity-physics-3d

name	unity-physics-3d
description	Unity 3D Physics

Unity 3D Physics

Use this skill when configuring Rigidbodies, colliders, joints, raycasts, or physics materials in Unity's PhysX-based 3D physics system.

FixedUpdate — The Physics Loop

All physics code goes in FixedUpdate, never Update.

void FixedUpdate()
{
    // Runs at fixed intervals (default 50Hz = 0.02s)
    // Use Time.fixedDeltaTime for calculations
    rb.AddForce(Vector3.forward * speed * Time.fixedDeltaTime, ForceMode.Force);
}

void Update()
{
    // Variable framerate — for input reading, visuals, UI
    // NEVER apply forces or move rigidbodies here
    float h = Input.GetAxis("Horizontal"); // Read input here
}

Timestep Configuration

// Project Settings > Time > Fixed Timestep
Time.fixedDeltaTime = 0.02f;   // 50 Hz (default)
Time.fixedDeltaTime = 0.01f;   // 100 Hz (more accurate, more CPU)

For vehicle or fighting games, consider 100 Hz. For casual games, 50 Hz is fine.

Physics Materials

Unity 6: \ was renamed to \ (with an 's') and \ was renamed to . The menu item is now Assets > Create > Physics Material.

Create via Assets > Create > Physics Material.

PhysicsMaterial mat = new PhysicsMaterial("Ice");
mat.staticFriction = 0.05f;     // Friction when stationary
mat.dynamicFriction = 0.03f;    // Friction when moving
mat.bounciness = 0.1f;          // 0 = no bounce, 1 = perfect bounce
mat.frictionCombine = PhysicsMaterialCombine.Minimum;   // How two materials combine
mat.bounceCombine = PhysicsMaterialCombine.Maximum;

collider.material = mat;

Combine Modes

Mode	Result
Average	(a + b) / 2
Minimum	min(a, b)
Maximum	max(a, b)
Multiply	a * b

Priority: Average < Minimum < Multiply < Maximum. The higher-priority mode wins.

PhysX Settings

Edit > Project Settings > Physics:

Setting	Default	Notes
Gravity	(0, -9.81, 0)	Change for moon/space games
Default Solver Iterations	6	Increase for stable joints/stacking
Default Solver Velocity Iterations	1	Increase for jitter-free contacts
Auto Sync Transforms	true	Set false for performance, call Physics.SyncTransforms() manually
Reuse Collision Callbacks	true	Reduces GC in collision callbacks

Performance Tips

1. Use primitive colliders over MeshColliders whenever possible
2. Pre-allocate raycast buffers with NonAlloc variants in hot paths
3. Disable Auto Sync Transforms and call Physics.SyncTransforms() once per frame
4. Increase Fixed Timestep (0.02 to 0.03) if physics load is high and precision is not critical
5. Use layers to minimize collision pair checks
6. Sleep thresholds — PhysX automatically sleeps idle bodies; don't wake them unnecessarily
7. Avoid GetComponent in collision callbacks — cache references in Awake

Topic Pages

• Rigidbody Fundamentals
• Applying Forces
• Common Gotchas