Physics Materials

Physics materials define how two colliding/contacting rigid bodies should behave. The physics material is assigned to a collider, allowing multiple materials to be used for a single rigid body. Each model contains a library of physics materials, which is empty by default. You can create new materials and assign them to various colliders. If a physics material isn’t set on a collider then it uses the default material. Please visit our API documentation sites for further details:

• Physics Material
• Physics Materials

Friction

Friction coefficients are used to determine the friction force between two contacting rigid bodies. ProtoTwin uses the Coulomb friction approximation, where the friction force is equal to the normal force multiplied by the friction coefficient. When the two contacting colliders use different friction materials, the friction coefficients are combined by multiplying them together.

ProtoTwin supports both static and dynamic friction coefficients. In general, it requires a greater force to start moving one body that is resting on another than to keep one body moving relative to another. Therefore, it is typical to set the static friction coefficient slightly higher than the dynamic friction coefficient.

Static Friction

The static friction coefficient specifies the friction coefficient for the material when the two contacting colliders are stationary relative to each other.

Dynamic Friction

The dynamic friction coefficient specifies the friction coefficient for the material when two contacting colliders are moving/sliding relative to each other.

Restitution

Restitution coefficients are used to determine the velocity of a rigid body after colliding with another rigid body. It can be considered an approximation of the elasticity of the material, where a coefficient of 1.0 specifies a perfectly elastic material. Note that the Bounce Threshold property of the physics settings will cause the restitution coefficients to be ignored for collisions where the relative velocity is smaller than the bounce threshold velocity.

Compliant Contacts

Physics material can be configured for compliant (soft) contacts. When the restitution coefficient is negative, the value is interpreted as a spring constant. Each contact point is then treated as a spring, where the damping coefficient can be specified to dampen the oscillations. This can be useful for approximating soft bodies, and for improving stability in some cases.