Aboxcar at a rail yard is set into motion at the top of a hump. the car rolls down quietly and without friction onto a straight, level track where it couples with a flatcar of smaller mass, originally at rest, so that the two cars then roll together without friction.
consider the two cars as a system from the moment of release of the boxcar until both are rolling together.
(a) is the mechanical energy of the system conserved?
(b) is the momentum of this system conserved?

next, consider only the process of the boxcar gaining speed as it rolls down the hump. consider the boxcar and the earth as a system.
(a) is the mechanical energy of the system conserved?
(b) is the momentum of this system conserved?

finally, consider the two cars as a system as the boxcar is slowing down in the coupling process.
(a) is the mechanical energy of the system conserved?
(b) is the momentum of this system conserved?