Friction fundamentals Which of the following statements about static friction are true under typical dry-contact conditions?

Introduction / Context:
Static friction prevents relative motion until a limiting value is reached. Understanding its direction, proportionality to normal reaction, and weak dependence on apparent contact area is essential in machine design, braking, and robotics.

Given Data / Assumptions:

• Dry, clean surfaces with no lubrication (Coulomb friction model).
• Normal reaction N acts perpendicular to the interface.
• Body is on the verge of motion (impending slip) or at rest.

Concept / Approach:
The Coulomb model states: maximum static friction F_s,max = μ_s * N. Below this limit, static friction adjusts to match the needed tangential force to prevent slip. Direction of static friction opposes the tendency (impending direction) of motion. The model is largely insensitive to the apparent area of contact; true micro-contact area scales with load and material properties.

Step-by-Step Solution:

Verification / Alternative check:
Place a block on a rough plane. Increase the applied tangential force slowly; friction matches it until F = μ_s N, then motion starts. Varying the block’s footprint at the same weight gives nearly the same limiting force, confirming area insensitivity.

Why Other Options Are Wrong:

• Any single choice: In isolation they are each true, but the option “all of the above” correctly captures the full behaviour described by the Coulomb model.
• None of the above: Contradicted by standard friction laws.

Common Pitfalls:
Confusing static with kinetic friction, and assuming friction always equals μ_s N even below the limit. Static friction takes whatever value is necessary up to μ_s N.

Final Answer:
all of the above