- Refinery (Hits: 433)
- Relativity (Hits: 245)
- Reservoir (Hits: 104)
- Resistance (Hits: 370)
- Rotating Equipment (Hits: 134)
- Rotational (Hits: 403)
- Roughness (Hits: 794)
- Safety (Hits: 2611)
- Science (Hits: 822)
- Sector (Hits: 1675)
- Segment (Hits: 1724)
- Separator (Hits: 2290)
- Soil (Hits: 13714)
- Solar (Hits: 205)
- Solid (Hits: 317)
- Solid Geometry (Hits: 298)
- Solid Prism (Hits: 571)
- Solid Pyramid (Hits: 607)
- Solid Wedge (Hits: 523)
- Specific Heat (Hits: 2005)