A 2.66 kg sphere makes a perfectly inelastic collision?

Question

with a second sphere that is initially at rest. The composite system moves with a speed equal to one-third the original speed of 2.66 kg sphere.

What is the mass of the second sphere (in units of kg).

Answer 1

Momentum is always conserved. In an inelastic collision, the second sphere sticks to the first.

Since momentum is mass * velocity, if the new speed is 1/3 the original, the new mass must be 3 times the original. So the second sphere weighs twice as much as the first.