Simple categories matching frictional:

• surface,
• coating,
• soil,
• system,
• material,
• substance,
• force,

Members with property frictional:

• abrasive_surface,
• rock,
• wind_resistance,
• ice,
• soil,
• geologic_material,
• sand,
• gravel,
• rubber,
• cam,
• bushing,
• bearing,
• capillarity,
• granular_material,
• air,
• air_resistance,
• fine_sand,
• bureaucracy,
• sand_paper,
• carpeting,
• plastic_foam,
• motor_torque,
• stone,
• clay,
• sponge_rubber,
• fabric,
• piston,
• steel,
• soft_rubber,
• chamois_skin,
• material_design,
• cork,
• friction,
• fps,
• dry_sand,
• water,
• unsaturated_soil,
• metal,
• clean_silt,
• grommet_strip,