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?

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

The process of water smoothing rough rocks is due to a phenomenon called abrasion. Abrasion occurs when water, carrying sediments or particles, flows over a surface and acts as a natural sandpaper. Here's a step-by-step explanation of how water can smooth rough rocks through the process of abrasion:

1. Introduction: Rough rocks have uneven surfaces with protrusions and rough edges, which can cause more friction against other objects.

2. Water as a lubricant: When objects come in contact and rub against each other, friction occurs. Friction is a force that opposes relative motion between two surfaces in contact. Water acts as a lubricant or a reducing agent for friction, meaning it helps to minimize the resistance between surfaces.

3. Sediments and particles in water: In natural environments, water is often not entirely clean and contains sediments, such as small rocks, sand, or minerals, suspended in it. These sediments act as abrasive particles.

4. Water flow: As water flows over rough rocks, it carries and transports these suspended particles. The flowing water exerts pressure and forcefully moves the sediments against the rocks' surfaces.

5. Abrasion process: The suspended sediments act like tiny tools or abrasives, rubbing against the surface of the rocks as they are transported by the water. Over time, the constant rubbing, grinding, and scraping motion caused by the moving particles against the rocky surface gradually wear away the irregularities.

6. Smoothing effect: As the sediments continue to interact with the rough rocks, they gradually remove the protrusions, irregularities, and other rough features. The repeated abrasion process results in a smooth surface.

7. Time and conditions: The rate at which water can smooth rough rocks depends on various factors such as the force of water flow, the size and sharpness of the sediments, and the duration of the erosion process. It may take years or even decades for noticeable changes to occur.

In conclusion, water can reduce friction between objects, but when water carries sediments that act as abrasives, it can also cause rough rocks to become smooth through the process of abrasion.

The process of water causing rough rocks to become smooth is known as erosion. Erosion is the gradual wearing away of rocks and other materials on Earth's surface. Here is a step-by-step explanation of how erosion can smooth out rocks:

1. Friction: When moving water comes into contact with rocks, even if they are rough, friction occurs. The force of the flowing water against the rock's surface causes particles to rub against each other.

2. Abrasion: The particles carried by the water, such as sand, pebbles, or sediment, act as natural abrasives. As they collide with the rock's surface, they chip away at the rough edges and gradually wear them down.

3. Continuous action: Over time, the process of water continuously flowing against the rocks causes more and more particles to collide with the surface. This repetitive action produces a smoothing effect, gradually wearing away the roughness of the rocks.

4. Velocity and turbulence: The speed and turbulence of the water also play a role in the erosion process. Faster-moving water carries more sediment, which leads to increased abrasion and faster wear on the rocks.

5. Geological time: It's important to note that these processes occur over long periods of time, often spanning thousands or millions of years. The smoothing effect on rocks is the cumulative result of countless interactions between water and the rock's surface.

In summary, the movement of water against rough rocks leads to erosion, causing the gradual wearing down of the rock's surface. This process is primarily driven by the friction between the water and the rocks, aided by the particles carried by the water as abrasives.