I can't solve questions related to conservation of momentum

[Capt1801]

I'm a passout from school taking a gap year. I find the concept of conservation of momentum exceedingly difficult. Each question - and sometimes each part of a question, if a question has different parts - requires us to choose different systems each time. I look at the solution, and think I have it now, and then I move to the next question only to find that I can't solve it. It seems as though questions from this chapter (which involves centre of mass and collisions too) confuse me. I just can't locate the doubt - I don't understand where I'm going wrong. I know why momentum is conserved. I know that in the absence of an external force, the velocity of the centre of mass is zero if it was initially at rest. I know the concepts, and understand them. And yet, when I get to the questions, I can't solve even one without getting the wrong answer at least once. I choose a particular system, equate the final and initial momentum and then get the wrong answer. Then I find I missed something in the system I chose, or chose the wrong system altogether.

Energy conservation is pretty simple as compared to this. My question is: is conservation of momentum actually so difficult or do I find it difficult because I'm missing something?

Thanks.

 

[Stephen Tashi]

My guess is that you find conservation of momentum difficult because you are missing something. However, we can't be sure about that based on your vague description of the problems you are asked to solve. Ask about a specific problem using the template for homework questions.