a carpenter vigorously rubs coarse sandpaper back and forth across a rough wooden plank. Wood shavings and dust particles fly off with each scraping motion. The wooden surface gradually becomes smoother where the sandpaper has worn away the rough spots. Before-and-after sections of the plank clearly show the transformation from bumpy to smooth. A workshop assistant nearby sweeps up the accumulated wood debris. Another worker examines a finished piece that has been completely smoothed. material being scraped and worn away by friction and repeated rubbing action