Question

A boxcar 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 mechanical energy of the system conserved? Yes No (b) Is momentum of this system conserved? Yes No 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. (c) Is mechanical energy of this system conserved? Yes No (d) Is momentum of this system conserved? Yes No Finally, consider the two cars as a system as the boxcar is slowing down in the coupling process. (e) Is mechanical energy of this system conserved? Yes No (f) Is momentum of this system conserved? Yes No

Answer 1

a) Yes, the mechanical energy of the system is conserved because they were no friction in the system.

b) Yes, the momentum of the system is conserved because the two car continued to move after collision and without friction.

c) No, the mechanical energy of the system is not conserved because the car is gaining speed due to gravitational force acting on the car as it roll down the hump.

d) No, the momentum of this system is not conserved because the car gains speed as it moves down, which changes the momentum of the car.

e) No, mechanical energy of the system is not conserved because the car is slowing down, that means frictional force is acting the oppose the motion of the car.

f) No, momentum of the system is not conserved because the velocity of the car has reduced after collision, which will cause the momentum of the system to change