Q. When a weight of 5 kg is suspended from a copper wire of length 30 m and diameter 0.5 mm, the length of the wire increases by 2.4 cm. If the diameter is doubled, the extension produced is
A) 1.2 cm
B) 0.6 cm
C) 0.3 cm
D) 0.15 cm
Explanation
(b) $Y = \ \frac{mg\ \times 4 \times l}{\pi D^{2} \times \ \mathrm{\Delta}l}\ or\ \mathrm{\Delta}l\ \propto \frac{1}{D^{2}}$
When D is doubled, $\mathrm{\Delta}l\ $becomes on- fourth, $ie,\frac{1}{4} \times 2.4\ cm,\ ie,\ 0.6\ cm.$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.