In engineering mechanics of friction, the term ‘‘dynamic friction’’ (also called kinetic friction) refers to the frictional resistance experienced by a body under which condition of motion or rest?

Introduction / Context

Frictional behavior is categorized based on whether contact surfaces are stationary or moving. Correctly identifying static, limiting, and dynamic (kinetic) friction is foundational for solving problems in brakes, clutches, and machine design.

Given Data / Assumptions

• Two solid bodies in contact with a normal reaction N.
• Classical Coulomb friction model.
• Uniform materials and clean, dry surfaces for conceptual clarity.

Concept / Approach

Static friction acts when there is no relative motion; it adjusts up to a maximum value to prevent slipping. The limiting friction is the threshold value at impending motion. Once sliding commences, the friction is termed dynamic (kinetic) friction, usually slightly less than the maximum static friction for the same normal load.

Step-by-Step Solution

Verification / Alternative check

Empirical data show μ_k (kinetic coefficient) is typically slightly smaller than μ_s (static), reflecting reduced interlocking during motion.

Why Other Options Are Wrong

• is at rest: describes static friction.
• just begins to slide: refers to limiting friction at the verge of motion.
• none of the above: incorrect because dynamic friction specifically corresponds to sliding motion.
• is vibrating but not translating: not a standard friction regime classification.

Common Pitfalls

• Using μ_s when the problem states sliding is already occurring; μ_k should be used.
• Confusing ‘‘limiting’’ and ‘‘dynamic’’ as synonyms; they are adjacent but distinct states.

Final Answer

is in motion