Question

water is useful for reducing friction between objects. However, you also learned that (due to friction) moving water causes rough rocks to become smooth. How can this be?

Answer 1

Water in rivers can carry rocks through the water and rub them against other rocks,which over a long period of time makes them smooth.

Answer 2

Water in rivers can carry rocks through the water and rub them against other rocks, which over long periods of time makes them smooth.