Afreight car with a mass of 10 metric tons is rolling at 108 km/h along a level track when it collides with another freight car, which is initially at rest. if the speed of the cars after they couple together is 36 km/h, what is the mass of the second car?
question options:

40 metric tons

20 metric tons

10 metric tons

5 metric tons (incorrect answer)

 m = 20 metric ton

Explanation:

Given,

The mass of the freight car, M = 10,000 kg

The velocity of the car before the collision, U = 105 km/h

The mass of another freight car, m = ?

The velocity of another freight car before the collision, u = 0 km/h

The velocity of the coupled car after the collision, V = 36 km/h

According to the conservation of momentum

                     MU + mu = MV + mV

                            mV = MU - MV                ∵   mu = 0

                            m = M(U - V)/V

                                 = 10,000 (108 - 36) / 36

                                  = 20,000 kg

Hence, the mass of the another freight car is, m = 20,000 kg

a and b sometimes have a positive value

0.11 m/s and 0.28 m/s should be correct