Help with Conservation of momentum.?

Question

I really need help with this question...

A railway carriage of mass 3.5 x 10^3kg is moving at 4.0m/s on a straight,horizontal track. It collides with a stationary carriage on the same track. As a result of the collision, the two carriages become linked together and move at 2.5m/s. Calculate the mass of the second carriage.

Answer 1

Initial momentum --> p1 = m1 v1

Final momentum ---> p2 = (m1+m2)*v2

m1 v1 = (m1+m2)*v2 ---> m1 v1/v2 = m1 + m2


m2 = m1*(v1/v2 - 1) = 3.5x10^3*(4/2.5 -1) = 2100 kg

Answer 2

.

Before collision.
Initial momentum of the carriage A = 3.5 x10^3 x 4 kg m/s
Initial momentum of the carriage B = 0.
Total momentum = 3.5 x10^3 x 4 kg m/s

After collision, let us suppose that both move in the initial direction of motion of A
Final momentum of the carriage A = 3.5 x10^3 x 2.5 kg m/s
Final momentum of the carriage B= M x10^3 x 2.5 kg m/s
Total momentum = (M+ 3.5) x10^3 x 2.5 kg m/s

Total momentum is conserved
(M+ 3.5) x10^3 x 2.5 = 3.5 x10^3 x 4
(M+ 3.5) = 3.5 x 4 /2.5 =5.6
M =2.1.

The mass of the carriage B = 2.1.x10^3 kg.