Physics

Problem

Three identical smooth spheres

X,\,Y,\,Z

are placed in a straight line on a frictionless horizontal surface with very small gaps between them. Sphere

X

is projected towards

Y

with speed

u

, while

Y

and

Z

are at rest. All collisions are perfectly elastic. After all possible collisions have taken place, which sphere(s) move(s) with speed

u

? a) Only

Y

b) Both

X

and

Y

with speed

u

c) Only

Z

d) All three together with speed

u/3