Question

A moving body of mass $m$ and velocity $3km/hr$. Collides with a body at rest and of mass $2m$ and then sticks to it. Now the combined mass starts to move, then the combined velocity will be :

• A. 4 km/hr
• B. 3 km/hr
• C. 2 km/hr
• D. 1 km/hr

Answer 1

Answer: (D)

Mass of body $\left(m_1\right)=m$;
Velocity of first body $\left(u_1\right)=3km/$ hour;
Mass of second body in rest $\left(m_2\right)=2m$
and velocity of second body $\left(u_2\right)=0$.
After combination, mass of the body $(M)=m+2m=3m$
From the law of conservation of momentum, we get
$Mv=m_1{u}_1+m_2{u}_2$
or $3mv=(m\times 3)+(2m\times 0)=3m$
or $v=1km/$ hour.