A string of length L and force constant k is stretched to obtain extension l. It is further stretched to obtainextension l1. The work done in second stretching is [MHT CET 2014]

Work done in stretching a string to obtain an extension I,W1=12kl2Similarly, work done is stretching a string to obtain extensionl+l1,  W2=12kl1+l2 Now, work done in second stretching,W2=W2'-W1=12kl1+l2-12kl2=12kl12l+l1