Question

Two balls are moving towards each other on a vertical line collides with each other as shown. Find the velocity of 4 kg ball just after collision, if the velocity of the 2 kg ball is 4 m/s upwards after collision.

• A. -3 m/s
• B. $\frac{1}{2}$ m/s
• C. 4 m/s
• D. $\frac{3}{2}$ m/s

Answer 1

Answer: (B)

$\frac{1}{2}$ m/s

Answer is B.
Let the final velocity of 4 kg ball just after collision be v. Since, external force is gravitational which is non - impulsive, hence, linear momentum will be conserved.
Applying linear momentum conservation
$2(-3)+4\left(4\right)=2\left(4\right)+4\left(v\right)$
or $v=\frac{1}{2}m/s$
Hence, the velocity of 4 kg ball just after collision is $\frac{1}{2}m/s$.