Kinematic pairs — a pair is known as a higher pair when the relative motion between the contacting elements is primarily of which type?

Introduction / Context: Kinematic pairs are classified as lower or higher based on the nature of contact. This classification affects wear, lubrication, and force transmission in mechanisms such as cams, gears, and rolling contacts.

Given Data / Assumptions:

• Lower pair: surface (area) contact with constrained relative motion (e.g., turning pair, sliding pair, screw pair).
• Higher pair: line or point contact; relative motion often involves rolling and/or pure sliding with minimal area contact (e.g., gear teeth, cam–follower).

Concept / Approach: In many textbooks and exam conventions, the hallmark of a higher pair is line/point contact with relative motion that is commonly treated as rolling at the contact (gears and pure rolling) rather than the full-surface sliding typical of lower pairs.

Step-by-Step Solution:

Verification / Alternative Check: Gear teeth have combined rolling and sliding but are treated as higher pairs due to line contact; exam standards often associate “rolling” with higher pair.

Why Other Options Are Wrong:
turning only — Revolute (lower) pair.
sliding only — Prismatic (lower) pair.
partly turning and partly sliding — Screw pair (lower).

Common Pitfalls: Assuming any line-contact with mixed sliding must be lower; classification is contact-based, not purely motion-component based.

Final Answer: rolling only.