Two balls, one of mass m and other of mass 2 m are moving towards each other with respective speeds

Two balls, one of mass m and other of mass 2 m are moving towards each other with respective speeds of 10 m/s and 20 m/s on a smooth horizontal floor. If after collision they stick to each other and move as combined mass (perfectly inelastic collision), then the speed of combined mass will be ?

1. 20 m/s

2. 10 m/s

3. 30 m/s

4. 50 m/s

Solution -

Applying conservation of momentum

10m-20(2m)=3m(v)

10m-40m=3mv

-30m=3mv

v=-10 m/sec

So, speed after the collision is 10 m/sec (<---------)

option (2) 10 m/s