Question

A ball is moving with a speed of $5\text{m/s}$ towards a heavy wall which is moving towards the ball with speed $2\text{m/s}$ as shown in figure. Assuming the collision to be perfectly elastic, find the speed of the ball and speed of the wall immediately after the collision.

• A. $9\text{m/s},3\text{m/s}$
• B. $7\text{m/s},3\text{m/s}$
• C. $9\text{m/s},2\text{m/s}$
• D. $7\text{m/s},2\text{m/s}$

Answer 1

The correct option is C $9\text{m/s},2\text{m/s}$

Given,
Mass of the ball $\left(m_1\right)$ is very small compared to the mass of the wall $\left(m_2\right)$.
i.e $m_1<<m_2$

Speed of ball before collision $\left(u_1\right)=-5\text{m/s}$
Speed of wall before collision $\left(u_2\right)=2\text{m/s}$
(rightward +ve)

Let the speed of the ball after collision $\left(v_1\right)=v\text{m/s}$
Speed of the wall after collision $\left(v_2\right)=v^{\prime }\text{m/s}$
In case of elastic collision, when $m_1<<m_2$
$v_1=-u_1+2u_2;v_2=u_2$
i.e $v=5+4=9\text{m/s}$
& $v^{\prime }=2\text{m/s}$ (both rightward)