Torsional and Rolling Friction

Two more kinds of friction: one resists an object spinning in place, the other resists it rolling across a surface.

Ordinary sliding friction resists two surfaces sliding across each other; there are two other kinds as well. Torsional friction resists an object spinning about the normal direction at a contact, which requires the contact to be a small patch of area rather than an ideal single point. Rolling friction (also called rolling resistance) resists an object rolling across a surface, coming mainly from the energy lost as the contact material repeatedly deforms (hysteresis), and it's usually much smaller than sliding friction — a steel wheel on a steel rail has a rolling-resistance coefficient of only about 0.0003–0.0004, versus about 0.01–0.015 for a car tire on concrete. In grasp analysis, a contact capable of providing torsional friction is called a soft-finger contact, since it can resist one more direction of torque than a plain point contact. Simulators expose these as tunable parameters: each MuJoCo geometry has three friction coefficients, in order sliding, torsional, and rolling, defaulting to 1, 0.005, and 0.0001; condim must be set to 4 to enable torsional friction, and to 6 to add rolling friction as well.

ExampleIn MuJoCo, gently push a ball resting on a plane: with condim set to 3 (sliding friction only), the ball keeps rolling indefinitely; with condim set to 6 and a rolling-friction coefficient specified, it gradually comes to a stop. Similarly, two fingers pinching a bottle so it can't twist between them are relying on torsional friction between the fingertips and the bottle's surface.

Also called

Spin Friction, Rolling Resistance

Related

Coulomb Friction · Friction Coefficient (Coefficient of Friction) · Friction Cone · Contact Model · MuJoCo (Multi-Joint dynamics with Contact) · Grasping

Sources

MuJoCo Documentation: Computation (Contact / Friction)
MuJoCo Documentation: XML Reference (geom friction, condim)
Wikipedia: Rolling resistance