Correct. I was only trying to answer the original poster's question. I apologize for not specifying that I was referring to closed systems when I said momentum is always conserved. Open systems are a different ball game. I jumped the gun by not reading the thread. Not to stroke your ego but I am consistently impressed by your replies, BRT. Every time I look through your posts critically for an error I never find one. It is exceedingly rare to find a person can explain physics conceptually without a loss of rigor.

Force is defined as delta-momentum/delta-time. In calculus that's expressed F=dp/dt where p is momentum. If you integrate force over time then you get the change in momentum and thus it can't be conserved. Let's say there is no force, F=0. That means that dp/dt = 0. If dp/dt = 0 then p is a constant because when you take the derivative of a constant you get zero. Saying momentum is a constant is equivalent to saying it is conserved! So momentum is conserved unless outside forces are involved...when outside forces are involved energy changes with time and it's a dissipative system.

If there is an outside force, let's say 5 newtons. then you have F=5N. dp/dt = 5N. Then if you rearrange this as 5dt = dp and integrate both sides you get p=5(tf-ti). This means the change in momentum is equal to 5(tf - ti) where tf is the ending time and ti is the starting time. In this situation momentum isn't conserved.

That should clear up why momentum isn't conserved in a system when you have an external force. A closed system is one where you have no external forces acting.

BRT, I just had an interview to teach for kaplan today. I suppose that makes us rivals?