Riley rubs two rocks together. she notices that some of the surface of one of the rocks was worn away to form small particles. what has happened to that rock? was it eroded or weathered?

Given that Riley is a human, it's strange to use the terms erosion or weathering to describe her interaction with rocks. Nevertheless, Erosion is the transport of surface materials (soil, rocks, etc.) through the actions of wind, water, and ice, while weathering is the physical/chemical breakdown or change of objects due to weather conditions occuring mainly through the action of wind, water, and temperature on rocks and soil. This coupled with the fact that erosion occurs over a distance makes the best answer weathering.