Laws of kinetic (dynamic) friction – conceptual check: Which statements correctly describe the behavior of kinetic friction between two surfaces in relative motion?

Introduction / Context:
Friction models are simplified but very useful for engineering calculations. The classical Coulomb model captures the first-order behavior of dry kinetic friction. Understanding these laws helps in analyzing brakes, clutches, belt drives, and any contact system experiencing sliding.

Given Data / Assumptions:

• Dry friction (no significant lubrication hydrodynamics).
• Surfaces and conditions are within moderate sliding speed ranges.
• Normal reaction N is well-defined and constant for the comparison.

Concept / Approach:
The basic kinetic friction model is F_k = μ_k * N, where μ_k is the coefficient of kinetic friction. Typically μ_k < μ_s (limiting or static coefficient). The friction force direction always opposes relative motion. Empirical trends show that μ_k is roughly constant at low to moderate speeds but slightly decreases as speed rises due to thermal and dynamic effects at the interface.

Step-by-Step Solution:

Verification / Alternative check:
Tribology data for metals and polymers commonly show μ_k lower than μ_s and often declining slightly with sliding speed until mixed or hydrodynamic regimes appear.

Why Other Options Are Wrong:

• “None” contradicts well-established dry friction behavior.

Common Pitfalls:
Assuming μ_k is independent of speed under all conditions; ignoring temperature rise, wear, or lubrication effects.

Final Answer:
All of the above