$K$ is the force constant of a spring. The work done in increasing its extension from ${l_1}$ to ${l_2}$ will be
A$K({l_2} - {l_1})$
B$\frac{K}{2}({l_2} + {l_1})$
C$K(l_2^2 - l_1^2)$
D$\frac{K}{2}(l_2^2 - l_1^2)$
Medium
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D$\frac{K}{2}(l_2^2 - l_1^2)$
d (d) At extension ${l_1}$, the stored energy $ = \frac{1}{2}Kl_1^2$
At extension ${l_2}$, the stored energy$ = \frac{1}{2}Kl_2^2$
Work done in increasing its extension from ${l_1}$ to ${l_2}$
$ = \frac{1}{2}K(l_2^2 - l_1^2)$
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