Sphere a Has Mass M and Is Moving with Velocity

Multiple Choice

Sphere A has mass m and is moving with velocity v. It makes a head-on elastic collision with a stationary sphere B of mass 2m. After the collision their speeds (v_A, and v_B) are:

A) 0, v/2
B) -v/3, 2v/3
C) -v, v
D) -2v/3, v/3
E) none of these