I'm confused about momentum, I thought that momentum was always 0, so how can it change?
GCSE Physics- I thought that momentum was always 0 because the momentum of objects cancel each other out e.g. someone firing a gun, the bullet has momentum, so does the person holding it, and it cancels each other out.
The total momentum of a system is the sum of the momentum of each element that forms part of the system. The total momentum of a system is not zero but constant.
As an illustration, consider the example in your question; initially both the gun and the bullet are stationary and have a total momentum that is zero. When the bullet is fired its momentum is not zero but it is the product of the mass of the bullet and its velocity. If the gun and the bullet are considered a closed system, the gun moves backwards when the bullet is fired because of the fact that the total momentum in the system has to remain constant. The momentum of the bullet as it moves in one direction is equal to the momentum of the gun but the two are in opposite directions thereby keeping the total momentum of the system constant. Here the total momentum of the system is always zero but this is not true for all systems.