Since there are no external forces, including friction, act on the flatcar. after the sack rests on the flatcar, we would assume that momentum is conserved. This means that
total momentum of car before collision = total momentum of car after collision.
Recall,
momentum = mass x velocity
From the information given,
mass of car before collision = 2000
velocity of car before collision = 3
Thus,
total momentum of car before collision = 2000 x 3 = 6000
Also,
mass of sack = 500
mass of car and sack after collision = 500 + 2000 = 2500
velocity after collision = v
momentum after collision = 2500 x v = 2500v
Since momentum is conserved, then
6000 = 2500v
v = 6000/2500
v = 2.4
the speed of the flatcar is 2.4 m/s
