3. I hold a bottle vertically, looking down at its cap. To open the bottle I rotate the cap
clockwise. This means the thread of the screw on which the cap moves is:
a. right-handed
b. left-handed
c. symmetric
d. this can’t be determined from the given information
Bottle caps use standard right-handed threads, where clockwise rotation tightens (or in this case, unscrews upward when viewed from above). This follows the “righty-tighty, lefty-loosey” convention for most screws and fasteners.
Question Breakdown
The scenario describes holding the bottle vertically and looking down at the cap, rotating it clockwise to open (unscrew) it. This action lifts the cap off the bottle neck, confirming a right-handed thread on the bottle’s neck, which the cap matches.
Option Analysis
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a. Right-handed: Correct. Right-handed threads slope upward to the right; clockwise rotation from above advances the cap upward (opening), as seen in standard bottles.
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b. Left-handed: Incorrect. Left-handed threads slope upward to the left; clockwise rotation would tighten (drive downward), not open the bottle.
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c. Symmetric: Incorrect. Screw threads have directional helices; symmetry would prevent reliable engagement or motion.
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d. Can’t be determined: Incorrect. The described rotation directly reveals the thread handedness based on standard mechanics.
Answer: a. right-handed
When you hold a bottle vertically, look down at the cap, and rotate it clockwise to open, the screw thread on the bottle neck follows right-handed convention. This standard design ensures intuitive use: clockwise unscrews upward from the viewer’s perspective. Right-handed threads dominate everyday items like bottles for ergonomic reasons tied to right-hand dominance.
Right-Handed vs Left-Handed Threads
Right-handed screw threads slope upward to the right when viewed from the end; clockwise turn advances them into or out of material. Bottle caps exemplify this: the neck’s thread is right-handed, so clockwise rotation disengages it. Left-handed threads, sloping upward left, require counterclockwise tightening and are rarer, used in high-vibration areas like bike pedals to prevent loosening.
| Thread Type | Slope Direction (Viewed from End) | Clockwise Rotation Effect (Top View) | Common Uses |
|---|---|---|---|
| Right-Handed | Upward to right | Tightens downward, unscrews upward on bottles | Bottles, screws, jars |
| Left-Handed | Upward to left | Loosens (moves upward) | Specialty tools, safety valves |
Physics Behind Bottle Cap Opening
Torque from fingers overcomes friction on the threads. Viewed top-down, clockwise torque on a right-handed thread produces upward linear motion via helical geometry. This matches human supination strength for tightening.
Exam Relevance for CSIR NET Physics
Questions on screw threads test mechanics concepts like helices and rotation direction. Visualize: thumb along axis, fingers curl clockwise for right-hand rule in threads. Practice identifies handedness instantly from motion description.
