A gun is mounted on a railroad car. The mass of the car, the gun, the shells and the operator is 50 m where m is the mass of one shell. If the velocity of the shell with respect to the gun (in its state before firing) is 200 m/s, what is the recoil speed of the car after the second shot? Neglect friction.
If the recoil velocity of car after first shot is v, applying conservation of momentum,
⇒ 0 = 49m×v + m×200
⇒ v = –200/49
After first shot, velocity of the car = -200/49 m/s
Similarly, after another shot, velocity of the car = -200/48 m/s
Total recoiling velocity of the car = 
m/s
(negative sign indicates that this recoil velocity is in a direction opposite to that of the shell)
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