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During a certain war, millions of shells were fired by country A towards the west, and even more shells were fired back by country B towards the east. The average momentum of each bullet were the same on average. What was the ultimate effect on the angular momentum of Earth?

Here's my try: B fired more bullets than A. Because average momentum of each bullet is the same, there must be a net change in momentum. As such the Earth's angular momentum would have to change.

However, the answer was that there was no effect, not even a very slight one. Why is that so?

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    $\begingroup$ This is homework, so I'm not going to answer. I'll instead give a hint, in the form of another question: What happens to the shells? $\endgroup$ – David Hammen Jul 3 '14 at 3:27
  • $\begingroup$ If they were fired into orbit you would have an interesting problem. $\endgroup$ – C. Towne Springer Jul 3 '14 at 3:37
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There is 1 reason; Newton's third law. When you fire a bullet, the bullet has a momentum in one direction (east) and the gun has momentum in the opposite direction (west). Of course, the person stops the gun from moving. When the bullet strikes an object, it imparts its momentum on the object. Neglecting air resistance, it is easy to show that all the forces cancel out and there is no change in momentum. Even with air resistance, the momentum doesn't change.

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