A railroad car of mass m and initial speed v collides with and becomes coupled with the two identical cars. Compute the final speed v′ of the group of three cars and the fractional loss n of energy if 

(a) The initial separation distance d = 0 (that is, the two stationary cars are initially coupled together with no slack in the coupling) 

(b) The distance d ≠ 0 so that the cars are uncoupled and slightly separated. Neglect rolling resistance.