A body of mass 1000kg is moving horizontally with a velocity 6m s ^...

From the conservation of linear momentum, it can be written that

$Mu = \left(M + m\right)v . . . \left(1\right)$

where, $M$ is the initial mass, $u$ is the initial velocity, $m$ is the extra mass added and $v$ is the final velocity of the composition.

From equation (1), it follows that

$1000 \times 6 = 1200 \times v ⇒ v = 5m{s}^{- 1}$