Physics Material

A data structure encoding surface interaction properties — friction coefficients, coefficient of restitution, density, and drag — that govern object behaviour within a physics simulation engine. Physics materials are distinct from visual rendering materials and are consumed by simulation solvers to produce physically plausible contact responses for metaverse avatars, props, and environments.

Semantic Classification

Content

Key Characteristics

• Surface Interaction: Defines contact behavior between objects

• Parameterized Properties: Numeric values controlling physics response

• Material Pairing: Combined properties determine interactions

• Simulation-Specific: Interpreted by physics engine

• Distinct from Visual: Separate from rendering materials

  Core Properties

  Friction Coefficients

  Static Friction (μₛ)

• Resistance to initiating motion

• Higher values = harder to start sliding

• Typical range: 0.0 (ice) to 1.5+ (rubber on concrete)

• Zero friction = frictionless surface

  Dynamic Friction (μₖ)

• Resistance during sliding motion

• Usually less than static friction

• Controls sliding speed and deceleration

• Typical range: 0.0 to 1.2

  Friction Combine Mode

• How to combine two materials’ friction values

• Average: (μ₁ + μ₂) / 2

• Minimum: min(μ₁, μ₂)

• Maximum: max(μ₁, μ₂)

• Multiply: μ₁ × μ₂

  Restitution (Bounciness)

  Coefficient of Restitution (e)

• Energy retained after collision

• Range: 0.0 (no bounce) to 1.0 (perfect bounce)

• Real materials: typically 0.1 to 0.95

• Values > 1.0 add energy (non-physical but useful)

  Restitution Combine Mode

• Average, min, max, or multiply

• Affects collision “bounciness”

• Important for consistent behavior

  Density

  Mass per Volume (ρ)

• Determines object mass if auto-calculated

• Measured in kg/m³ or g/cm³

• Typical values:

• Wood: 500-700 kg/m³

• Metal: 2700-8000 kg/m³

• Rubber: 900-1200 kg/m³

• Water: 1000 kg/m³

  Affects

• Gravitational force (weight)

• Momentum and collision response

• Floating/sinking in fluids

  Drag & Damping

  Linear Drag

• Resistance to linear motion through medium

• Air resistance coefficient

• Higher values slow objects faster

  Angular Drag

• Resistance to rotational motion

• Spinning objects slow down over time

  Advanced Properties

  Surface Softness

• Deformation under pressure

• Contact area calculation

• Affects collision detection precision

  Sound Properties

• Impact sound selection

• Volume based on collision force

• Frequency based on material type

  Thermal Properties

• Heat conduction (for advanced sims)

• Specific heat capacity

• Thermal expansion

  Electrical Properties

• Conductivity

• Dielectric constant

• Useful for specialized simulations

  Material Combination

  Pairwise Interaction

  When two materials collide, combined properties determine behavior:

Combined Friction = CombineMode(friction₁, friction₂)
Combined Restitution = CombineMode(restitution₁, restitution₂)

Common Presets

Material	Static μₛ	Dynamic μₖ	Restitution	Density (kg/m³)
Ice	0.05	0.03	0.1	917
Wood	0.6	0.4	0.2	600
Metal	0.7	0.5	0.3	7850
Rubber	1.2	1.0	0.8	1100
Concrete	0.8	0.6	0.1	2400
Glass	0.5	0.4	0.7	2500
Cloth	0.4	0.3	0.0	200

Implementation in Physics Engines

Unity PhysicsMaterial

PhysicMaterial myMaterial = new PhysicMaterial();
myMaterial.dynamicFriction = 0.6f;
myMaterial.staticFriction = 0.7f;
myMaterial.bounciness = 0.3f;
myMaterial.frictionCombine = PhysicMaterialCombine.Average;
myMaterial.bounceCombine = PhysicMaterialCombine.Maximum;

Unreal Engine Physical Material

UPhysicalMaterial* MyPhysMat = NewObject<UPhysicalMaterial>();
MyPhysMat->Friction = 0.6f;
MyPhysMat->Restitution = 0.3f;
MyPhysMat->Density = 1000.0f; // kg/m³
MyPhysMat->FrictionCombineMode = EFrictionCombineMode::Average;

PhysX Material

PxMaterial* material = physics->createMaterial(0.5f, 0.5f, 0.1f);
// Parameters: staticFriction, dynamicFriction, restitution

Applications

Metaverse Interactions

• Avatar walking on varied terrain (grass, ice, wood)

• Object manipulation (picking up, throwing)

• Vehicle handling (tires on different surfaces)

• Climbing and parkour mechanics

  Game Mechanics

• Puzzle design (sliding ice blocks)

• Platforming (bouncy mushrooms)

• Sports simulation (ball physics)

• Combat (weapon impacts)

  Training Simulations

• Realistic tool interaction

• Material handling training

• Safety scenario rehearsal

• Heavy machinery operation

  Virtual Production

• Physical plausibility for pre-visualization

• Stunt planning and rehearsal

• Prop interaction planning

  Design Considerations

  Realism vs. Fun

• Real-world values may not be enjoyable

• Exaggerate for clarity and feel

• Consistency more important than accuracy

  Performance

• Physics materials add computation

• Limit complex interactions for performance

• Use simplified models for distant objects

  Authoring Workflow

• Pair visual materials with physics materials

• Provide sensible defaults

• Allow per-instance overrides

• Preset library for common materials

  Challenges

  Material Authoring

• Expertise required for realistic values

• Iterative tuning for desired feel

• Testing across diverse scenarios

  Interoperability

• No universal standard for physics materials

• Manual mapping between engines

• USD PhysicsMaterial schema (emerging standard)

  Simulation Stability

• Extreme values cause instability

• Very high/low friction can break solver

• Restitution > 1.0 adds energy (divergence risk)

  Best Practices

• Start with realistic reference values

• Tune iteratively through playtesting

• Maintain material library with presets

• Document material choices and rationale

• Use combine modes consistently across project

• Test material pairings explicitly

• Consider performance implications

  References

• NVIDIA PhysX Documentation: Materials and Surfaces

• Hecker, C. (1997). “Physics, Part 3: Collision Response”

• Erin Catto (2005-2023). Box2D Documentation

• Millington, I. & Funge, J. (2009). Artificial Intelligence for Games, 2nd ed.

Provenance