During (that is, from before contact until after contact) an isolated, totally elastic two-body collision, the pair of parameters that remains unchanged is
A) the center-of-mass velocity and the center-of-mass speed.
B) the center-of-mass speed and the speed of either body with respect to the center of mass.
C) the speed of either body with respect to the center of mass and the velocity of either body with respect to the center of mass.
D) the velocity of either body with respect to the center of mass and the center-of-mass velocity.

Question

Quizplus · Accepted Answer

In an isolated, totally elastic two-body collision, both the center-of-mass velocity and the center-of-mass speed remain unchanged. This is because the total momentum of the system (which determines the center-of-mass velocity) is conserved, and the total kinetic energy (which influences the center-of-mass speed) is also conserved in an elastic collision.

During (That Is, from Before Contact Until After Contact) an Isolated