A block of mass 10 kg, moving in x direction with a constant speed of 10 ms^–1, is subjected to a retarding force F = 0.1 x J/m during its travel from x = 20 m to 30 m. Its final KE will be : 

475 J

W = - ∫ F dx

F = 0.1 x J/m

⇒ W = - ∫₂₀³⁰ 0.1 x dx

W = - 0.1 \([\frac{x^2}{2}]_20^30\)

W =- 25 J

W = K_f - K_i

K_i = \(\frac{1}{2}(10)(10)^2\)

K_f = 475 J