Friction occurs when two objects come into contact with one another. Friction is a force that works in the opposing direction than an object is moving. The smoother the surfaces of the two objects in contact, the less friction. The rougher the surfaces of the two objects, the more friction. Think of a car on the road: when the driver of a moving car hits the brakes, the car’s slower-rolling tires against the ground create more friction and that friction helps the car stop. If the car is driving on water or ice, there is less friction, and thus it takes the car longer to stop, if the car does not start to slide. If sand or gravel is placed on icy roads, that makes the surface of the road rougher and therefore helps to create friction, allowing cars to stop more easily on an icy road.
An interesting fact about friction is that the amount of frictional force created by two objects in contact with each other is independent of the surface area of the objects. The amount of friction created is based on the how rough or smooth the objects are, not on the size of the objects. So two objects that are the same size could create the same amount of friction as two objects of different sizes, as long as the surfaces are the same in texture.
This is not the case related to an object’s weight. If a one-pound brick is placed on the floor next to a three-pound brick, the frictional force of the three-pound brick is greater. That is because the weight or load of the brick on the floor is also a type of force.
There are different types of friction. Kinetic friction is force created by moving objects—like sliding a brick across the floor. Static friction is the amount of force it takes to keep items in place—like a parked car. Rolling friction occurs when a ball or cylindrical object rolls across a surface—like rolling a marble on a table.
The question “What are the effects of rougher surfaces on friction?” has a simple answer if the weight and shape (round or flat) of the objects is constant—the rougher the surfaces, the more friction. If the weights and/or shapes of the objects differ, then mathematical equations come into play and the answer is not quite so simple, because the type of friction and additional types of force come into consideration.