Friction and slopes — definition check “The angle of an inclined plane at which a body just begins to slide down is called the helix angle.” Is this statement correct?

Introduction / Context:
This item tests terminology in engineering mechanics and machine design. Two different angles are often confused: the angle of repose (slip threshold on an incline) and the helix angle (geometry of helical features like screw threads).

Given Data / Assumptions:

• A rigid block rests on a rough inclined plane.
• We slowly increase the plane's angle until impending motion occurs.
• Basic Coulomb friction model applies.

Concept / Approach:
The angle of repose is the inclination at which a body is just about to slide. At this angle, tan(φ) = μ_s, where μ_s is the coefficient of static friction. The helix angle is a geometric angle formed by the helix (e.g., a screw thread) with respect to a plane perpendicular to the screw axis; it is unrelated to a block beginning to slip on a plane.

Step-by-Step Solution:

Verification / Alternative check:
Practical test: pour granular material to form a conical pile; the slope angle at rest is the angle of repose, again demonstrating the correct term.

Why Other Options Are Wrong:

• Only for screws / polished planes / friction zero: These do not change the definition; helix angle remains a geometric feature, not a slip threshold.

Common Pitfalls:
Mixing terms due to superficial similarity. Always connect “repose” with sliding onset and “helix” with helical geometry.

Final Answer:
False