What type of friction acts on objects that are not moving?

Static friction is the type of friction that acts on objects that are not moving. It is the force that resists the initiation of sliding motion between two surfaces in contact. When an external force is applied to an object at rest, static friction responds by increasing, up to a certain maximum limit, to prevent the object from moving.

This force is crucial in everyday situations, such as pushing a heavy box or starting a car from a stationary position, where the static friction allows these actions to occur without slippage until the force exceeds the static friction threshold. Once the applied force surpasses this limit, the object will begin to move, at which point kinetic friction takes over.

In contrast, kinetic friction comes into play once an object is in motion relative to another object. Rolling friction occurs when an object rolls over a surface, like a wheel on the road, while fluid friction refers to the resistance encountered by objects moving through a fluid (liquid or gas). Since the question specifically asks about objects that are not moving, static friction is the clear and correct choice.