Which type of friction occurs when two surfaces are not in relative motion?

Static friction occurs when two surfaces are in contact but not sliding against each other. It is the force that must be overcome to start moving an object at rest. This type of friction is crucial in various applications, such as ensuring that a stationary object remains at rest until sufficient force is applied to set it in motion.

The value of static friction can vary depending on the materials involved and the forces acting upon them, allowing for a range of resistance up to a certain maximum threshold, beyond which the object will start to move and kinetic friction will take over. Understanding static friction is essential for tasks ranging from simple everyday actions to complex engineering challenges.

In contrast, kinetic friction refers to surfaces sliding relative to each other, rolling friction deals with objects rolling over surfaces, and fluid friction refers to the resistance encountered by objects moving through fluids. Each of these types of friction has its own characteristics and applications, but none applies to the scenario where there is no relative motion between the surfaces involved.