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# A spring with a spring force constant $k=800\; N/m$ has a length of 5 cm. The work done in extending from 5 cm to 15 cm is

$(a)\;16\;J \quad (b)\;8\;J \quad (c)\;32\;J \quad (d)\;24\;J$

The work done by the spring force depends only on the end points. If the block is moved from an initial displacement $x_i$ to a final displacement $x_f$, the work done by the spring force $W_s$ is $\large\frac{1}{2} $$k (x_i^2 - x_f^2) \Rightarrow W = \large\frac{1}{2}$$\times 800 \times (15^2-5^2)$$= 8J$
edited Aug 25, 2014