Q. An elastic string has a length $l$ when tension in it is 5 N. Its length is $h$ when tension is of 4 N. on subjecting the string to a tension of 9 N, its length will be
A) $l + h$
B) $l - h$
C) $(5\ l - 4\ h)$
D) $(l + h)/(h - l)$
Explanation
(c) If $L$ is the original length of spring, and $k$ is a spring constant of the spring, then
$L + (5/k) = l$ …(i)
and $L + (4/k) = h$ …(ii)
$\therefore\ \ \ \ \ l - h = 1/k\ \ \ or\ \ \ \ k = 1/(l - h)$
and $L = (5h - 1)$
$\therefore$ Length of spring when subjected to tension 9 N is
$= L + 9/k = (5h - 4l) + 9(l - h)$
$= (5l - 4h)$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.