A wire of cross-section $4 mm^2$ is stretched by 0.1 mm by a certain weight. How far (length) will be wire of same material and length but of area $8 mm^2$ stretched under the action of same force.
$ l = { FL \over AY } $ $ \therefore l \alpha { 1 \over A} $ ( F , L and Y are constant ) $ { l_2 \over l_1} = { A_1 \over A_2} = {4 \over 8} = {1 \over 2} \Rightarrow l_2 = { l_1 \over 2 } = { 0.1 \over 2 } = 0.05 mm $