Question

Two identical balls collide with each other and then with the wall. All the collisions are elastic. Identify the correct statement(s).

• A. Velocity of the ball $A$ after all collisions is $10\text{m/s}$ leftwards.
• B. Position of ball $B$ after all collisions is $10\text{m}$ from wall.
• C. Velocity of ball $A$ after all collisions is $5\text{m/s}$ leftwards.
• D. Position of ball $B$ after all collisions is $5\text{m}$ from wall.

Answer 1

Answer: (C), (D)

Position of ball $B$ after all collisions is $5\text{m}$ from wall.

We know that in elastic collision, two identical bodies exchange their velocities.


After first collision, ball $A$ will come to rest and ball $B$ moves with $5\text{m/s}$ towards the wall.
$t=\frac{5}{5}=1\text{s}$ is the time taken for first collision and ball $A$ will be at $5\text{m}$ from wall.

Next, ball $B$ will collide with the wall and rebound with speed $5\text{m/s}$.
After second collsion of ball $B$ with ball $A$, exchange of velocities will occur.
Ball $B$ will come to rest and ball $A$ moves with $5\text{m/s}$ towards left.
So finally, ball $B$ will be at a distance $5\text{m}$ from wall.
Option (c) and (d) are correct.