Why is the momentum of a system conserved?

[piareround]

Hey guys I was helping my roomate study for a physics test:

A block of mass M is at rest over a small hole in a table. A bullet of mass m is fired directly upward into the block, striking it with a speed vb. It passes through the block, emerging with a speed vb. To what maximum, h, does the block rise above the table? Express your answer in terms of parameters defined in the problem and physical or mathematical constants.

Here is the question:
In the problem above, what is true while the bullet is passing through the block?

Here is the solution:
a) Momentum in the bullet-block system is nearly conserved, as the interaction forces are large compared to the external forces.


I found this usually because I thought in all case's except maybe for something involving virtual particles, momentum was always conserved whether it was only linear or angular; it was energy that sometimes was not conserved; however, the answer here suggested it not.

Does anyone know why? Can someone give another example where momentum is not conserved?

 

[Doc Al]

The momentum of a system is conserved in the absence of external forces. But here there are external forces, namely gravity. Nonetheless, you can ignore gravity when treating the collision between bullet and block.