An engine is attached to a wagon through a shock absorber of length...

Work $= \DeltaK . E .$

${W}_{\text{triction}} + {W}_{\text{spring}} = 0 - \dfrac{1}{2}m{v}^{2}$

$- \dfrac{90}{100}\left(\dfrac{1}{2}m{v}^{2}\right) + {W}_{\text{spring}} = - \dfrac{1}{2}m{v}^{2}$

${w}_{\text{spring}} = - \dfrac{10}{100} \times \dfrac{1}{2}m{v}^{2}$

$- \dfrac{1}{2}k{x}^{2} = - \dfrac{1}{20}m{v}^{2} ⇒ k = \dfrac{40000 \times ( 20{)}^{2}}{10 \times ( 1{)}^{2}} = 16 \times {10}^{5}$