• # question_answer 35) A block attached to a spring, pulled by a constant horizontal force, is kept on a smooth surface as shown in the figure. Initially, the spring is in the natural state. Then the maximum positive work that the applied force F can do is (given that string does not break) A)  $\frac{{{F}^{2}}}{K}$                                  B) $\frac{2{{F}^{2}}}{K}$C)  $\infty$                                  D) $\frac{{{F}^{2}}}{2K}$

Solution :

For maximum positive work the final speed must be zero. Let the extension in the spring be 1. Applying work-energy theorem on the block,  we get$F\ell -\frac{1}{2}k{{\ell }^{2}}=0$ $\ell =\frac{2F}{k}$or work done $=F\ell =\frac{2{{F}^{2}}}{k}$ Hence, the correction option is [b].

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