Question

A moving body of mass $m$ and velocity $3\, km / hr$. Collides with a body at rest and of mass $2\, m$ 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

The Correct Option is D

Here : Mass of the body $m_{1}=m$
Velocity of the first body $u_{1}=312\, m / \sec$
Mass of second body at rest $m_{2}=2\, m$
Velocity of second body $u_{2}=0$
After combination total mass of the body
$M=m+2m=3m$
Now from the law of conservation of momentum
$M v=m_{1} u_{1}+m_{2} u_{2}$
(where $M=m+2 m=3 m$ )
$3 m v =m \times 3+2 m \times 0$
$v =1\, km / hr$