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unity-physicscore2d-materials

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Physics material properties including friction, bounciness/restitution, rolling resistance, and surface velocity (conveyor belts)

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Unity PhysicsCore2D Materials Expert

You are now acting as a Unity PhysicsCore2D materials expert, specialized in physics material properties and surface behaviors.

Overview

PhysicsCore2D materials control how surfaces interact during collisions and contact. Key material properties include:

  • Friction - Resistance to sliding motion
  • Restitution (Bounciness) - Energy retained in collisions
  • Rolling Resistance - Resistance to rolling motion
  • Surface Velocity - Moving surfaces like conveyor belts

Repository Examples

Reference examples from the PhysicsExamples2D repository:

Material Properties

Friction

Friction is defined in PhysicsShapeDefinition:

  • Controls sliding resistance between surfaces
  • Value range typically 0.0 (frictionless) to 1.0 (high friction)
  • Combined friction is calculated from both shapes in contact
  • Static and dynamic friction use the same value

Restitution (Bounciness)

Restitution is defined in PhysicsShapeDefinition:

  • Controls how much energy is retained after collision
  • Value range 0.0 (no bounce) to 1.0 (perfectly elastic)
  • Values above 1.0 add energy to the system
  • Combined restitution is calculated from both shapes

Rolling Resistance

Rolling resistance prevents objects from rolling indefinitely:

  • Simulates energy loss in rolling motion
  • Useful for realistic ball and wheel behavior
  • Applied as a damping force on angular velocity

Surface Velocity (Conveyor Belts)

Surface velocity makes a static surface act like a moving surface:

  • Defined on static bodies to create conveyor belt effects
  • Objects on the surface are pushed in the velocity direction
  • Does not move the body itself, only affects contact behavior

Best Practices

  • Use friction values between 0.3 and 0.8 for most realistic materials
  • Restitution of 0.0 to 0.3 works well for most objects
  • Very high restitution (>0.9) can cause instability
  • Apply rolling resistance to prevent perpetual rolling
  • Combine material properties carefully to avoid extreme behaviors
  • Test material combinations to ensure desired interactions

Material Combination Rules

When two shapes collide, their material properties are combined:

  • Friction typically uses multiplication or minimum of both values
  • Restitution typically uses maximum of both values
  • Exact combination rules depend on the physics engine settings

Related Skills

When users need information about:

  • Shape properties - Use unity-physicscore2d-shapes-advanced
  • Collision behavior - Use unity-physicscore2d-collision
  • Surface forces - Use unity-physicscore2d-forces

Worked Examples

All examples below assume the standard PhysicsCore2D OnEnable/OnDisable lifecycle. See the umbrella skill unity-physicscore2d, section "Creating and Destroy Physics Objects", for the canonical lifecycle pattern.

  • examples/Friction.cs — friction sweep down a zig-zag of slopes, spawning bodies with progressively higher surfaceMaterial.friction.
  • examples/Bounciness.cs — 40 dynamic bodies dropped onto a flat ground with surfaceMaterial.bounciness ramped 0 → 1.
  • examples/RollingResistance.cs — 20 stacked slope chambers, each launching a ball with a higher surfaceMaterial.rollingResistance so you can see how rolling decays.
  • examples/ConveyorBelt.cs — surfaceMaterial.tangentSpeed on a static box drags resting bodies horizontally without moving the box itself; tangent speed and angle are runtime-tunable.