How does the speed change when 4000kg are added to the inital mass of 10,000?

Question

A 10,000 kg railroad car is rolling at 3.0 m/s when a 4000 kg load of gravel is suddenly dropped in. What is the car's speed just after the gravel is loaded?

Answer 1

Mass * Velocity = momentum

10000kg* 3m/s = 30000 = M1

M2 has to equal M1

14000*v2 = M2 = M1

14000*v2 = 30000

v2 = 2.14m/s

Answer 2

V = 3*10000/14000 = 2.14 m/s