Question

A ball is thrown horizontally at a heavy wall with a velocity of $15\text{m/s}$. The wall is moving in the same direction as the ball with a velocity of $8\text{m/s}$. Assuming that the collision is elastic, what is the velocity of the ball after the collision?

• A. $1\text{m/s}$
• B. $2\text{m/s}$
• C. $7\text{m/s}$
• D. $8\text{m/s}$

Answer 1

The correct option is A $1\text{m/s}$

In an elastic collision, the relative velocities of the bodies before and after the collision remains the same. Since the wall is heavy, we can assume that the velocity of the wall remains the same before and after the collision.
$u_1=15\text{m/s},u_2=8\text{m/s},v_2=8\text{m/s}$
Consider the direction of the ball to be positive direction.
For an elastic collision, we have,
$u_1-u_2=v_2-v_1$
$\Rightarrow 15-8=8-v_1$
$\Rightarrow 7-8=-v_1$
$\Rightarrow v_1=1\text{m/s}$
Hence the correct option is (a).