6. A billiard ball traveling at 4.0 m/s has an elastic head-on collision with a billiard ball of equal mass
that is initially at rest. The first ball is at rest after the collision. What is the speed of the second ball
after the collision?
A) 4.0 m/s
B) 8.0 m/s
C) Not enough information to solve this problem
D) 4.0 kg m/s​