A machine gun is attached to a railroad flatcar that rolls with negligible friction.If the railroad car has a mass of 7 × 10⁴ kg,how many bullets of mass 20 g would have to be fired at 200 m/s off the back to give the railroad car a forward velocity of 0.5 m/s?

Question

Quizplus · Accepted Answer

We can use conservation of momentum in this problem where the momentum of the bullets is equal and opposite to the momentum gained by the railroad car. 
Let v be the velocity of the railroad car after firing the bullets. Then, 
(7 × 10^14 kg) (v) = (20 g) (N) (200 m/s)  where N is the number of bullets fired
Solving for N, we get:
N = (7 × 10^14 kg)(0.5 m/s) / (20 g)(200 m/s) = 8,750 
Therefore, the answer is D, 8,750 bullets.

A Machine Gun Is Attached to a Railroad Flatcar That