Question

On a smooth surface there are five identical equally spaced balls A,B,C,D and E present with initial velocities 10m/s, -5m/s, 2m/s, -3m/s, 3m/s respectively, Collision between any two ball assumed to be elasitc. Then after all possible collision which pair of balls will have same speed as of initial

• A. A & D
• B. B & C
• C. C & D
• D. A & C

Answer 1

Answer: (C)

C & D

In this question we must consider the fact that in final state, the most negative velocity will be possessed by ball A and most positive by ball E since the only they won't collide ever. Hence finally $V_A=-5m/s$ and $V_E=10m/s$. Also, from similar argument, $V_B=-3m/s$ and $V_D=3m/s$. Left is $V_C=2m/s$.

Hence, speeds of balls C and D will remain same.