A frictionless puck of mass 1.0 kg travels east at 90 m/s toward an initially motionless puck of mass 2.0 kg. When the pucks are 12 meters apart, the speed of the center of mass of the system is
A) 30 m/s.
B) 40 m/s.
C) 60 m/s.
D) 80 m/s.
E) 90 m/s.

Question

Quizplus · Accepted Answer

The total momentum of the system is conserved, so the center of mass will move at a constant velocity. The initial momentum of the system is 90 kg m/s to the east. The final momentum of the system will be the sum of the momenta of the two pucks, which will be 180 kg m/s to the east. The center of mass will therefore move at a velocity of 180 kg m/s / 3 kg = 60 m/s to the east.

A Frictionless Puck of Mass 1