When does static friction occur?

• A. When objects are sliding past each other
• B. When objects are in motion with constant speed
• C. When two surfaces are at rest and not sliding
• D. When objects are in free-fall

Answer: (C)

Static friction occurs when two surfaces are in contact but are not moving relative to each other. This force acts to oppose the initiation of motion between the surfaces. In this scenario, the surfaces are at rest, and static friction is what prevents them from sliding past one another under the influence of an applied force.

In contrast, when objects are sliding past each other, dynamic or kinetic friction comes into play, which is distinctly different from static friction. Objects in motion with constant speed are also not experiencing static friction because they are already in motion, which removes the conditions necessary for static friction to act. Lastly, in free-fall, objects are not in contact with a surface that would create a frictional force, thereby also eliminating the possibility for static friction to occur.