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A string is wound over a circular disc. It is now unwound with its “free end” letting it remain tight and tangent to the disc. The curve traced by the “free end” is
  • a)
    Spiral
  • b)
    Involute
  • c)
    Cycloid
  • d)
    Epicycloid
Correct answer is option 'B'. Can you explain this answer?
Most Upvoted Answer
A string is wound over a circular disc. It is now unwound with its &ld...
Involute
  • Involute is the curve traced by a point on a chord as it unwinds (but remains tight) around a circle
  • Alternatively, an involute may be defined as the curve traced by a point on a straight line which rolls around a circle or polygon without slip
  • Depending on the plane shape around which the line rolls, the involutes are named as involute of a triangle, involute of a square, involute of a polygon, involute of a circle etc
  • Application: The most common application of involute is seen in the manufacture of gears. The profile of a gear tooth is the shape of an involute
Cycloid: When the generating circle rolls on a straight line, the path of the point is a cycloid.
Epicycloid: When the generating circle rolls on the outside (convex side) of a larger circle, the path of the point is an epicycloid. ​
Hypocycloid: When the generating circle rolls on the inside (concave side) of a larger circle, the path of the point is a hypocycloid. 
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A string is wound over a circular disc. It is now unwound with its &ld...
Understanding the Involute Curve
When a string is wound around a circular disc and unwound while maintaining tension, the path traced by the free end of the string forms a specific geometric shape known as an involute.
Key Characteristics of the Involute:
- The involute of a curve is generated by tracing the end of a taut string as it unwinds from the curve.
- For a circular disc, the involute is derived from the circular path of the disc.
- As the string unwinds, it remains tangent to the disc at the point of contact.
Why the Involute?
- The involute reflects the relationship between the length of the string and the radius of the disc.
- The unwinding process keeps the string straight and tight, creating a unique spiral-like path that diverges from the disc.
- As the radius of the disc decreases (due to unwinding), the involute continues to extend outward.
Visual Representation:
- Imagine the disc as a circle and the string as a line that starts from the edge of the circle.
- As you pull the string away from the disc, the path it traces is not simply circular but forms a spiral-like curve that is characteristic of an involute.
Conclusion:
The curve traced by the free end of the unwound string is an involute, showcasing the elegant geometric properties of curves and the relationship between tension and motion. Understanding this concept has applications in various fields, including mechanical engineering and design, particularly in the analysis of gears and belts.
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