Question

Balls of masses $4\text{kg}$ and $10\text{kg}$ are moving towards each other with speeds $6\text{m/s}$ and $3\text{m/s}$ respectively on a frictionless surface. After collision, the $4\text{kg}$ ball is observed to return back (towards left) with speed $2\text{m/s}$. Find the velocity of $10\text{kg}$ ball after collision.

• A. $2\text{m/s}$
• B. $0.2\text{m/s}$
• C. $9\text{m/s}$
• D. $1\text{m/s}$

Answer 1

The correct option is B $0.2\text{m/s}$

Given,
$m_1=4\text{kg};m_2=10\text{kg}$
$u_1=6\text{m/s};u_2=-3\text{m/s}$
$v_1=-2\text{m/s}$
(taking rightward +ve)
Let velocity of $m_2$ after collision be $v_2=v\text{m/s}$

Using principle of conservation of linear momentum,
$m_1{u}_1+m_2{u}_2=m_1{v}_1+m_2{v}_2$
$4\left(6\right)+10(-3)=4(-2)+10\left(v\right)$
$\Rightarrow v=0.2\text{m/s}$ (towards right)