Two blocks, each of mass m, moving in opposite directions with the same speed u, on a horizontal frictionless surface collide with each other, stick together and come to rest. Then,
Work done by external force on the system is zero.
Work done by the internal force on the system is -
Pi = mu - mu =0, Pf= 0, as they come to rest.
⟹Fext =0
⟹Wext =0
⟹ Kf−Ki = (0 −(12mu2+12mu2)) =-mu2