A disc is hanging from a thin rod as shown in the figure below. The rod of length ‘L’ is fixed at the top and connected to the disc at the bottom having radius ‘r’. The twist angle of the disc can be reduced by:
- AIncreasing the length ‘L’ of the rod
- BDecreasing the modulus of rigidity in the rod
- CIncreasing the radius of the disc
- DNone of the aboveCorrect
Solution
Concept
Twist angle
Formula
θ = TL/GJ
Step-by-step solution
1. θ = TL/GJ: increases with L and decreases with G and J. 2. Increasing L raises θ; decreasing G raises θ; the disc radius does not enter the rod's twist.
Explanation
None of the listed actions reduces the twist.
Why the other options are wrong
A and B increase the twist; C has no effect.
Shortcut
θ ∝ L/(G J).
Correct answer: D) None of the above