Definition check: What is kinetic friction? Choose the most accurate description for kinetic (dynamic) friction acting between two solid surfaces.

Introduction / Context:
Friction is commonly categorized into static and kinetic types. Static friction applies up to the onset of sliding; kinetic friction applies once sliding occurs. Accurate definitions help avoid conceptual errors in block-on-incline, belt friction, and brake problems.

Given Data / Assumptions:

• Two dry, rough, solid surfaces in contact.
• Neglect lubrication, wear, and temperature effects.
• Classical Coulomb friction model applies.

Concept / Approach:
Static friction F_s adjusts up to its limiting value F_lim = μ_s * N just before motion begins. Once the bodies are in relative motion, kinetic friction F_k = μ_k * N acts, typically with μ_k < μ_s. The problem asks specifically for the definition of kinetic friction.

Step-by-Step Solution:

Verification / Alternative check:
In experiments, the pull required to keep a block sliding at constant speed is less than the maximum pull needed to start motion, reflecting μ_k < μ_s.

Why Other Options Are Wrong:

• “Just about to move”: Defines limiting static friction.
• Angle definition: That is the angle of friction (an angular representation), not kinetic friction.
• Ratio of limiting friction to normal reaction: That is the coefficient of static friction μ_s.
• None of these: Unnecessary; one option is correct.

Common Pitfalls:
Using μ_s after sliding has begun; forgetting that kinetic friction is usually lower than the peak static value.

Final Answer:
Friction acting when the body is in actual motion