A Freight Car Rolls Along a Horizontal Track at 12

Multiple Choice

A freight car rolls along a horizontal track at 12 m/s and hooks onto an identical freight car at rest. The coupled cars will

A) come to a quick stop,
B) roll at 3 m/s,
C) roll at 6 mph,
D) roll at 12 ms,
Which makes sense because
E) in the absence of external forces, trains in motion continue in motion.
F) the acceleration of the car at rest equals the force of impact divided by the two masses.
G) since twice the mass is moving, the speed will be half for the same momentum.
H) energies before and after the collision remain the same.