General question about linear momentum.

[-Dragoon-]

This is the one thing I seem to still be very confused about. From what I understood, momentum is conserved if the net force on an isolated system, that is \frac{dP}{dt}, is equal to 0. But in some problems that I've worked through, I've always assumed the force due to gravity causes a change in momentum and thus the momentum of the system is not conserved, but my TA for some reason told me to just "ignore gravity" and always assume momentum is conserved.

My question: If momentum is conserved only if the rate of change of momentum with respect to time (net force) is equal to 0, then why is it for some problems the book and my TA's proceed to solve a problem where an object is in free-fall as if momentum is conserved? Doesn't gravity cause a change in momentum?

 

[gneill]

My question: If momentum is conserved only if the rate of change of momentum with respect to time (net force) is equal to 0, then why is it for some problems the book and my TA's proceed to solve a problem where an object is in free-fall as if momentum is conserved? Doesn't gravity cause a change in momentum?

It might help if you could give an example of such a problem. There may be particular circumstances that allow conservation of momentum to be used (such as two objects interacting over a vanishingly small time interval at a given elevation in the gravity field).